Oh man this is not a computer guy! nevertheless ill try to help. HOw much? im Guessing ( i could be wrong) 50-60$. you need PC3200 DDR 400 ram.
If you know a computer smart friend, see if they can test the ram for you in a PC. They like to act REAL dumb with bad ram.
This involves taking the side panel off, and the ram is alittle stick held in by 2 clips.
But just for stupidity sake, see if this happens when you move it to a different outlet.....seriously, ive spent 3 weeks working on a server, replaced EVERYTHING including motherboard, turns out the outlet had a bad ground. ugh did that SUCK. the comp would just run sometimes for 3 minutes, other times an hour or 2.
Also, does the freezing happen more often the mroe you use it? Like will it work for 20minutes, then like 7 or 8, then like 3 or 4? COuld be a heat issue....not saying YOU did anything, but maybe a fan is weak or a heatsink is loose