Well, DDR400 will work with your motherboard, but it'll run at DDR333.

Something like this should work fine in your system.

Basically, for either 512MB or 1GB sticks, you want 184 pin DDR SDRAM DDR333 (also known as PC2700) which runs at 2.5v, as that's the industry standard for DDR RAM. Avoid any DDR2 RAM, as it won't work!

There's not really a great deal more to consider, except that some older motherboards can be very picky about mixing RAM brands, or even different lines from the same manufacturer's product range. Generally speaking, though, it isn't a major issue - I never had any problems with my old Socket-A motherboard and mixed brands. Do pay attention though to the CAS latencies, as different RAM performs at different speeds, and the system BIOS may get confused by different CAS latencies on the RAM, and set the latencies too low for one of the sticks. If there is a problem, it will manifest itself through random freezes and restarts, usually, but may also lead to application errors. If this happens, setting the latencies manually in the BIOS to match the slowest RAM module can fix the problem.

If you have any more questions, feel free to ask