Uhhh, Radeon DDR
If you want the cost for around $100 then go with the GF2MX, but for $50 more the 32MB Radeon DDR is a
much better board. Great 2D on nearly all monitors, best video playback of any consumer video card, excellent gaming performance and feature support.
Lemme think(these numbers are off the top of my head so no flames please)-
PS/Case ~$50
Duron 750 w/heatsink and fan~ $100
Asus A7V mobo ~$130
256MB PC 133 RAM ~$120
Radeon 32MB DDR ~$150
Seismic Edge Soundcard ~$50
I know you said that you were keeping many of the parts, and I upped the RAM and CPU a bit(I would reccomend strongly going with 256MB. RAM is so cheap now it is very little extra in terms of cost and already games are using far more then 128MBs), but that comes in at ~$600 and that should be fairly close to current market prices.