1. Get a good PSU - it really makes a lot of difference
2. Don't buy anything important from Ebay - there is a reason that RAM is cheap...
3. Strongly consider buying an AXP-M and running it at 2.5GHz
Going with a S754 system still limits your future upgrade options, unless you are looking to invest in a Semip0rn system down the road. By the time 64bit capability is more than a novelty, you will be well past the point of upgrading anyway all that hardware. You're not gonna build a system with tremendous longevity for $900 these days.
If you replace your CPU and MB choices with an Abit NF7-S ($83) and a 35W Mobile Barton ($87), you will save $167 which you can put toward something else, like an X800! I can pretty much guarantee an X800 + Barton will outperform an A64 3000 and a 9800Pro in any game you are likely to enjoy playing. Even if you stick with the 9800Pro, you could get some really HQ RAM and still have a chunk of pocket money left over.
I'm not sure about your aversion to overclocking, but from a technical standpoint, it really couldn't be easier with the mobile barton. It's already unlocked, just spend 2 minutes in BIOS to change the multiplier to 12.5, the FSB to 200, and the voltage to 1.8 or 1.9. Everything else will run spec and with some sweet 2-2-2-10 RAM you'll have a killer system. No need for watercooling, or even a humongous HSF that runs at 50dB! That's the beauty of using mobile chips...
Just think about what you really want to do with this system, and what is really not important. If you want to play with beta builds of Windows64 then maybe you should lay out the extra $167. Even in a year's time 32-bit software and hardware will be far more prevalent than their 64-but counterparts.
If you must go the 64-bit route, then you need to suck it up and realize that it just costs more. But if you're gonna skimp, don't do it on the RAM. RAM is one of the most important compnenets of your system for so many reasons. Take your $790 and get a single 512MB DIMM of Mushkin PC3200 2-2-2 for $167. Yes, it is expensive, but it is the fastest (or almost) RAM you can buy at PC3200, which is perfect for you since you don't overclock. S754 is a single-channel system anyway, so you aren't penalized for having only one DIMM, and you can always add more later. If you must stay close to $900, then go for OCZ Enhanced Latency Series 184 Pin 512MB PC-3200 Platinum Edition for $118. Damn good memory, and your only $8 over the $900 mark. Don't get obsessed with having 1GB RAM. The performance difference is minimal, and it is MUCH better to have less HQ RAM then more LQ RAM.