http://tw.giga-byte.com/MotherBoard/Products/Products_GA-7VAXP.htm
Type : DDR333 (PC2700) / DDR266 (PC2100) / DDR200 (PC1600) -184pin
Max capacity : Up to 3GB by 3 DIMM slots
You can use any size of 184 pin DIMM up to 1GB (1024MB) in size. You can also use PC3200 DDR (DDR400), but your mobo will only run DDR at a max of 333MHz (non-OC'd of course), so basically, if you get PC3200 or faster DDR (not DDR2 mind you), it will clock down the speed of the RAM to the fastest the board can take or the slowest RAM stick that's in the mobo already. So in your case, You already have a Corsair 512MB PC3200 stick, if you got another stick of PC3200, it'll run both at 333MHz.
It doesn't really matter what brand you get, just get a good name brand (like Corsair) that's at least PC2700 (DDR333, PC3200 is probably the most common so might as well get it) and you shouldn't have any problems upping your memory.
Tavis Curry