I'm trying to fix up my sisters old Intel computer and found some old ram sticks kicking around so I wanted to put them into her computer to add a little more juice! But after researching her motherboard on the internet (It's an Aopen AX4GE-N), I found that it only supports up to DDR333 [PC2700]! So if I were to put the PC3200 ram in, would the motherboard just slow it down and make it run at 333Mhz (from 400Mhz) or would I just fry it??
There would be absolutely no problem. The PC3200 (DDR400) will run as PC2700, and indeed since it is underclocked it may be possible to reduce the latency timings. There are some incompatibilities among early DDR but that shouldn't apply to PC3200.