Even before the CPU upgrade arrived, I knew I wouldn't be satisfied, and I changed my plan to a fresh build and ordered a bunch of new stuff. When everything showed up and I got a system together, I realized I had a powerful machine held back by a video card that wasn't up to the same standard as everything else. Even worse, the 9600 non-pro is locked at a crippled setting. I installed a patch to overcome this, but it was buggy.
Because of these mistakes, I spent approximately $185 I could have spent on something else... but, I've got the components and will use them to improve my old system.
Expensive Mistakes: I started out with the plan of upgrading my existing system, a two-year-old budget barebones with a Chaintech board and an older VIA chipset. To upgrade it on the cheap, I ordered a 2.6ghz Celeron to replace the 1.7ghz Celeron and an ATI 9600 non-pro All-in-Wonder card.
I replaced the AIW with a 9800 pro, and now everything clicks. I used Earth 2160 as my performance tester. It ran just well enough to play on the 9600, but it runs fast and smooth on the new card at default settings.
My MSI board came with documentation that explains each BIOS setting. It's easy to adjust frequencies, voltages, etc. It is a socket 754, Nforce 3, AGP board, an excellent choice for my needs but not one I would recommend except to people who understand and accept what they are giving up. My philosophy was to go cheap and get the most for my money now, instead of worrying about a future upgrade path. I also tend to like products that are late in their cycle, because they entail less risk of technical problems that haven't been addressed.
I chose a high-quality power supply and put it in a server case I got from the landfill. I mounted a 92mm fan on the side, directed at the video card, and left the case open. I bought a couple of wire grates at the hardware store (sold for cooling pies and cakes) and fastened them to the sides of the case to provide some protection for the components inside. The grate on one side is fastened with binder clips for quick access. Everything runs nice and cool.

I have overclocked my system using my time-honored approach, which is to advance the settings until the machine blue screens, and then back off to the highest stable setting. I can't say I know what I'm doing, but I have faith, and that's what really counts.
At some point I will probably double my RAM to 2gb. I'm happy with a 40gb IDE hdd, because I recently bought a 300gb external drive for storage. If I bought a new internal drive, I'd go with SATA, but the performance difference isn't enough to motivate me to upgrade that aspect of my system.
I used stuff I already had where possible, but most of the system is newly purchased from a number of vendors, depending on who offered the best deal. Here is a breakdown of components and costs, minus the parts I bought but didn't use:
APPROXIMATE
COST INC. SHIPPING RUNNING TOTAL
Newly purchased:
Gfx card (ATI radeon 9800 pro/128mb) $135 $135
Power supply (Antec TP II 380) 60 195
RAM (2x512mb Corsair Value Select) 80 275
Board (MSI K8N Neo2/nForce 3) 55 330
CPU (Athlon 64 3000+) 125 455
DVD-R/RW drive (BenQ LightScribe) 80 535
Firewire card/cable (Startech) 30 565
Monitor (Dell 15" LCD, used) 120 685
OS (Windows 2000 Professional sp4) 85 770
Heat sinks (RAM & Video RAM) 20 790
Maxtor 300gb USB/firewire external storage drive 200 990
From existing system:
HDD (40gb WD IDE)
Optical mouse (Belkin)
Salvaged:
Case
FDD
Keyboard (IBM [mfr. by Lexmark] from 1993, old favorite)
Soundcard (Sound Blaster Live, SB0100, plus Live Drive 5-1/4" front console)
Speakers (Quick Shot Sound Mate II)
Misc. cables & fasteners