The nforce series boards based on the nvidia chipset are very picky about the type/make and quality of ram used on them. I've seen many many posts about the Kingston (Value) ram causeing many issues like random shutdowns, failed boots, unstable system...ect on nforce boards regardless of who the manufacture is.
Basicly on these boards (I have one) cheap ram=stability issues so you need quality ram. Kingston value ram...is not it. There HyperX though...works well as it's timings are tighter. I would also suggest a 400watt PSU or better for any nforce board running an AMD XP cpu...as these boards are power hungry.
Right now.....no game uses the full 128MB...but that could change later. Doom3 may be close...but I can't remember the minimum system requirments (think it was 128MB)..but I'm not sure. Halflife2 won't be that high.