ASUS CPU Support page for the A7V8X
As you can see from the above link, your system will support up to an Athlon XP 3000+ if your system's southbridge has the FSB333 sticker, or an 2800+ if not. PCProgress has the XP 2800+ in stock (I think they're used) for $60. You may find a better/riskier deal on EBay.
The specifications state you can have three DDR266 DIMMs, two DDR333 DIMMs or only one DDR400 DIMM installed. What route you take will depend on what's currently installed or if you're willing to replace what's there. RAM upgrades would run between $25 (single DDR266 512 Meg) and $100+ (three DDR266 1GB) depending on what you decide to do.
Your graphics card is about as high as I'd go with your system.