I'm also in the process for upgrading right now. In terms of upgrades, there are several ways to go. You can:
1) Go Low-End. This is good in the sense that you will have the highest value/performance ratio.
2) Go Mid-Range. This is most of us. And is good for those who have a need for something higher end, but don't want to throw away their money needlessly.
3) Go High-End. This is good for people who either do not want to upgrade for a long time or have money to burn. You will get the best of the best, but in 6 months time, your computer will have gone down in price from 1/3-1/2 it's original cost.
Depending on what you are upgrading from, you might not have a need to upgrade to Dual-Core. Right now, not many programs take advantage of the second core and there are a lot of good deals on single core Athlon 64 processors and s939 motherboards. Also, by using a s939 you can save by using the less expensive DDR memory.
However, if you plan to upgrade this system in a year or two, you may run into problems finding upgradable parts and in which case, you may have to pay a premium to salvage the board and memory as the manufacturers shift production to DDR2 memory and AM2 motherboards.
However, you could take a gamble and hope that in a years time, the price on dual-core s939 chips will go down. At the moment, you can get a dual-core Opteron for as low as $170.00. This might go down in the future, or because of scarcity, it may remain the same in a years time. It's hard to say, but it's your call.
Now, if you're looking to go dual core right now, then go Intel and buy Core 2 Duo. Currently, while AMD x2 processors are selling for less, the price difference isn't enough to not justify the advantages of getting a faster Core 2 Duo processor. The only
downside with going Intel is the high price of quality motherboards and as said before, the high cost of DDR2 memory.
If you're worried about the viability of the Socket 775/T platform, it's been documented that the new crop of Intel Quad Processors will be supported on the S775 platform. But you should check your motherboard manufacturer to make sure. But off the top of my head, the popular Asus P5W-DH will support the quad core Intel processor, though that's something I read second hand from someone on the forum, so don't take my word.
Anyway, if you want to go high end, then also go Intel. The Intel Core 2 Duos are the current CPU kings and if you're going to spend, then spend for the best.
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If you're upgrading your video and your budget is in the $200 range. At the top of my list you should upgrade in the following order based on cost from lowest to highest.
ATI Radeon X800GTO - Around $90-$100
Nvidia 7600GT or Ati X850XT/PE - Around $130-$150
Nvidia 7900GS or ATI x1950Pro - Around $180-$200.
For under $100.00 there isn't any retail card beside the X800GTO that is faster in gaming. You might be able to find some better deals on the forum like the ATI X850XT for $100AR, but those were then and not now.
For the Sub-$150, the ATI X850XT is an older card then the 7600GT and will beat out the Nvidia card in most games, but in at higher resolutions and under certain games, they perform similarly. But the X850Xt takes up two slots, needs more power, and some complain about the sound of the fan.
For the Sub-$200 range, you can get the Nvidia 7900GS and some can be had for around $180, but you can also get the new ATI x1950PRO, which replaces the X1900GT for around $200. In fact, when the Asus X1950PRO first showed up on NewEgg, it was only $165.00 but has since risen up to $190.00, with the Sapphire one at $200.00.
Between the two cards, the x1950Pro is better overall.
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If you're concerned about future upgradability, then don't go with s939 because no one knows what the future holds. However, if you're the type who will not save the motherboard or CPU or even the RAM when you upgrade and don't care about the advantages of single versus dual core, then go with an Athlon 64 FX-55 from NewEgg for
$140.00 (See HotDeals Forum for details). You can get a MSI nForce4 Ultra board at the Geeks for $41.00 that could over clock this chip really high. Plus if you have any DDR memory already you can use this in the system. But if not, you save about $30/GB between buying DDR versus DDR2.
Anyway for around $200.00 you got yourself an awesome single core system. For $200.00, you are only at the half-way point between buying a retail Intel e6300 ($170) or e6400 ($220). And to get a worthwhile motherboard that will overclock, it will cost about $140-$150 for say a Gigabyte P965-DS3 mobo. And you're going to be spending anywhere from $30-$60 more for 1-2GB of Memory. So by going Intel, you've have spent about $140 to $230 more going Intel versus going AMD using a s939 board.
So, assuming you want to upgrade your system in the future, you could use the the savings of $140-$230 to apply that towards a new dual-core system or use the extra savings now and buy a better video card.
In a years time, I can see prices of the Intel e6300 or e6400 going down about $50.00 to around $120/$180. Hopefully in a years time, Intel boards would have matured enough that we could see good quality boards for around $100. And hopefully DDR2 prices will return to normal, to around $80-$100/GB. If that happens, then if you decide to upgrade, you can get the Dual-Core system for around $300-$380. If you sell your parts from the s939 system (Board, RAM, CPU) I outlined, you may be able to recoup enough money (Say $150-$200), plus the money you saved going with single core ($140-$230), to pay for the dual-core upgrade a year from now.
So in other words, if you're frugal and are not already upgrading from a highend a64 system, then with a little effort on your part you can save enough money now to pay for a future upgrade later w/ you only spending about $190-$230 in the end, versus paying out $340-$430 to get a Dual Core system now. A potential savings of $150-$200--that's enough for a second video card to go SLI/Crossfire or to buy a Raptor 150GB.