On the last computer I built, I used two ($120 each) Geil 512 meg sticks, and when XP installer was copying files from the CD, it hung at close to the same place every time. Ran memtest, got tens of thousands of errors on one of the sticks, all within the 411 and 412th megabytes. Had to RMA both sticks and waited an extra 2 weeks to get the computer running. So yeah, it sounds like the problems you are having could very well be from the ram.
On a side note, I'm building a computer for my girlfriend right now, and got a 512mb stick of 35 dollar Rosewill ram. Memtest shows no errors, even with OC'd speeds and timings. Take what you want from that.