I picked up some DDR2 5300 that was on sale, and found that it would not physically fit on my motherboard. Now I've found some PC2 4200 RAM on the internet for the price I want. My motherboard takes PC 3200. Should I buy the PC2 4200?
DDR2 is not interchangeable/compatible with DDR memory. Your motherboard will only take DDR memory, and the memory you linked to is DDR2 memory, so no, it will not work. You need to find some DDR. Your motherboard is spec'd for DDR400, but it would be OK to get faster DDR RAM; it would just run at 400MHz, so you wouldn't get any speed improvement. Google Product Search might be a good starting place.