You have an AGP x8 slot like me. If you want to have any decent level of gaming for today's games on AGP, I highly recommend any variant of the HD 3850 AGP.
I run the best one which is HIS Radeon HD 3850 512MB 256-bit GDDR3 AGP 4X/8X HDCP Ready IceQ 3 Turbo Newegg.com - HIS Hightech H385Q512ANP Radeon HD 3850 512MB 256-bit GDDR3 AGP 4X/8X HDCP Ready IceQ 3 Turbo Video Card - Desktop Graphics / Video Cards
I highly recommend this one since it is well overclocked from the factory and you can manually overclock it a lot more. Well worth the price if you do not want to do a full system upgrade.
Find the drivers you need now here (you might have to try the XP ones, under download tab): ASUSTeK Computer Inc.
Do a Windows Update to see if places the correct drivers for you first, to get the sound up and running.