I would personally get an X800PRO/X800GTO/X800XL or 6800GT/6800GS AGP used on ebay for like $30 (agp versions of course). These top end cards from the 6 an x8 series will cream a mid range x1 series and probably be cheaper too: btw 6800GT and X800XL are the fastest. X800XT is better but more expensive (these cards are 16 pipes, the 6800GS/X800Pro/X800Gto are 12)
http://cgi.ebay.com/BFG-NVIDIA-GeForce-6800-GT-OC-256MB-AGP-Video-Card_W0QQitemZ220238704083QQihZ012QQcategoryZ3762QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/EVGA-e-GeForce-6800-DX-Video-Card-VGA-and-DVI-NR-AGP_W0QQitemZ260244182362QQihZ016QQcategoryZ3762QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/EVGA-nvidia-GeForce-6800-GT-256mb-GDDR3-AGP-8X_W0QQitemZ270240136360QQihZ017QQcategoryZ3762QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/ATI-Radeon-X800-Pro-256MB-AGP-Video-Card-NO-RESERVE_W0QQitemZ290232732279QQihZ019QQcategoryZ3762QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
for some reason there are few Ati's up right now. I had an X800XL and it played my COD2, BF2, and Source games quite nicely.
That said they aren't amazing, so newer games will require some modesty. If you can get an 2600xt, x1950pro, x1950gt, or 7800GS (agp versions of all), those would be great step up if you can get it for like $80. Otherwise try and find a cheap 3850 agp, even if it will be bottlenecked a bit, it still kicks.
I just noticed you have an LGA775 board, so you can upgrade to a PCI-E board without chaning CPU's. You just need to grab some cheap ram with it, but you can bump up to like 2gb for $25
http://cgi.ebay.com/Gigabyte-GA-G31MX-S2-G31-775-mATX-Motherboard-NEW_W0QQitemZ250249878964QQihZ015QQcategoryZ108978QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
that would be a good option if you can get it for under $50
or a cheapy ECS for like $35-40
http://cgi.ebay.com/Brand-New-ECS-G31T-M-G31-mATX-Motherboard-CPU-NIB_W0QQitemZ150251535702QQihZ005QQcategoryZ108978QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
http://cgi.ebay.com/NEW-ECS-G31T-M-LGA-775-Intel-G31-Micro-ATX-Motherboard_W0QQitemZ300228096679QQihZ020QQcategoryZ108978QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
2gb ram for $25 with rebate
http://www.newegg.com/Product/Product.aspx?Item=N82E16820220174
tack on a 2600XT PCI-E for $50
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161176
or a 3850 pci-e for $100
http://www.newegg.com/Product/Product.aspx?Item=N82E16814131096
I love my 3850, it maxes all my games, including bioshock and frontlines. 2600xt will be a little less than 1/2 as fast.
This gives you the option to upgrade to Core 2 Duo, Core 2 Quad, or Pentium Dual Core (great cheapy CPU for overclocking) and is really a good investment. Also uses PCI-E for future video cards. Still, if you just need a cheap upgrade drop $30 for a 6800/x800 agp.
BTW a 7600GT is faster than an x800xl most of the time. It will put the x1650's head to the sidewalk and execute it while ninja fighting uma thurman with a katana. They are counters parts in that they are mid ranged cards from corresponding generations, but the x1800gto was the real counter to the 7600GT.