The rig that I ordered yesterday is as follows:
ASUS A8N-SLI Deluxe Motherboard
AMD Athlon 64 3000+ (Winchester) Processor
Arctic Silver 5 Thermal Compound
Zalman Aluminium/Copper LED CPU Heatsink/Fan
Crucial Ballistix 512MB DDR PC-3200 RAM (2)
BFG GeForce 6800 GT OC PCI Express Video Card
Maxtor DiamondMax 10 200GB Hard Drive
Lite-On 16X DVD-ROM Drive (Black)
NEC 16X Double Layer DVD±RW Drive (Black)
SONY Floppy Drive (Black)
Enermax Noisetaker 470W Power Supply
Antec Super LANBOY ATX Mini Tower Case
Vantec Nexus Fan/Light Controller (Black)
Vantec Spectrum PCI Fan Card
$1400 in total.
I've done over two months of research, so you can trust that these parts have been scrutinized time and time again. For your situation, if you're not getting a whole new system like I did, then here's what you need to be concerned with from what I can tell that you say you need to purchase for an upgrade: motherboard, CPU, RAM, GPU, hard drive, PSU (?).
Here are my suggestions for you with subsequent explanations:
ASUS A8N-SLI Deluxe Motherboard. It is nForce 4 SLi compatible, and in my opinion, this or the DFI nF4 SLi-D/R are the only options for motherboards right now on the AMD64 platform. $199.99 shipped from ZipZoomFly.com
AMD Athlon 64 3200+ (Winchester) Processor. Best price/performance ratio, and you may be able to find the OEM version of it on MonarchComputer.com for $179.00 shipped. If it is still $195.00 shipped, like I think it is, then you may be better off going with the 3000+.
Arctic Silver 5 Thermal Compound. You cannot do without this. $9.99 shipped from ZipZoomFly.com
Zalman Aluminium/Copper or Pure Copper (LED) CPU Heatsink/Fan, or comparable product. The one I got, model #: CNPS7000B-AlCu LED, has an LED fan and is much lighter than the copper version, though it cooler less efficiently. I am a frequent LAN-goer, so my rig is very light. The tradeoff is that it will not OC as much as I'd like it to. If you have a computer with a static location ([joke]no static electricity, mind you[/joke]), then go ahead and get the pure copper version of this, but make sure the model # says "CNPS7000B-", or "CNPS7700B-"; not "CNPS7000A-", or "CNPS7700A-". Alternatively, you could try Swiftech, Thermalright's XP-90/120 (again, size and cost are factors), or Thermaltake.
BFG GeForce 6800 GT OC PCI Express Video Card. $399.99 shipped from Chumbo.com. OC'ed right out of the box with a lifetime warranty. Unless Gainward makes a 6800 GT OC PCI Express, BFG will be the king of 6800 GTs.
Maxtor DiamondMax 10 200GB Hard Drive. Speeds rivaling WD Raptors with more than twice the capacity for about 4/5 of the price. A steal at ZipZoomFly.com for $121.00 shipped. NCQ-, SATA, and SATA II-compatible.
Enermax Noisetaker 470W Power Supply. 20- or 24-pin motherboard compatible, supports SATA, cheaper than the comparable NeoPower or OCZ PowerStream. Get it for $80.99 shipped from ZipZoomFly.com. Whatever you get, make sure it's 450W or higher (NVIDIA's official minimum requirement for their 6800 series).
Other possible upgrades include:
Vantec Nexus Fan/Light Controller (Black). I purchased an Antec Super LANBOY, which is notorious for less-than-adequate ventilation, so I got a fan controller to hopefully ramp up the fan speeds, as well as the extra 80 or 92mm top vent fan that I plan on modifying the case to fit.
Vantec Spectrum PCI Fan Card. I highly recommend this for aesthetic looks, but more importantly, the 6800 series runs exceptionally hot, and the BFG 6800 GT OC PCI Express GPU has been known to run as hot as 80C under full load. I bought this for the aforementioned reasons and again, because the Antec Super LANBOY has poor ventilation (from what I've heard).
Those are my suggestions, which were all very well thought out and implemented in my own rig. From the other posts I read on this thread, I would like to say that the vanilla 6800 is a piece of crap -- get a 6600 GT, or better yet a 6800 GT. The minimum PSU wattage needs to be 350W for the 6600 GT, and 480W for the 6800 series, as quoted from NVIDIA, though 450W will make do for most. Socket 754 is a waste of time and money -- I can't stress this enough. A major exception would be the DFI LANPARTY nF3 250gb coupled with an Athlon 64 3400+, though I still feel that it is sorely outdated, if not already outdated by socket 939. Newcastle is a waste of time and money as well. Go with Winchester for the AMD64 3000+ through 3500+. Any higher than 3500+, and you're talking MAJOR bucks and different CPU core manufacturers (Clawhammer and Sledgehammer, which are both extremely respectable). The 90nm manufacturing process is undeniable. Even Intel, slow, lumbering, monopolizing, awful, awful Intel has moved to the 90nm process in their Prescott line, though Prescotts still run unacceptably hot. AMD's advantages are cooler CPUs, which enable very flexible OC'ing, and price. If you construct an AMD64 rig, please do yourself a favor and take advantage of this. OC'ing is not terribly difficult and it is one of the most economical actions that you can take with regards to computer hardware.
Hope this helps.
Good Luck.