is it the motherboard, ram or video card?

my computer began having problems about two days ago.. i don't know what's up. i haven't done anything different. pls help?

my computers about 2 years old, with a 256 ram processor, an nvidia vid card with 128 memory, and about three fans.

i've got big software installed in there like photoshop, indesign, illustrator, dreamweaver, and all those web design software.. but of late, when the problems started to happen, i was mostly using word (ms office).. i've been writing some seo copy.

anyway, my symptoms are:
1. monitor would turn black and turn back on. the cpu would be on and running, even though the monitor turns black.
2. sometimes the monitor will totally just shut down, while cpu still runs. (so i'd restart)
3. when monitor blacks out, it looks like it freezes. what you type doesn't show in the document until, say, a minute.
4. monitor's pretty steady when you're not doing anything. like when i just leave it there and i just stare at it bewildered.
5. i've tried another monitor and it happens with it too so it's not a monitor problem.
6. i've reset the video card -- thinking that maybe it just needs tightening. only to find that it still happens. so either i didn't tighten it enough or it's not the vid card. frankly, i dont know how much tighter it can go

so.. is it th vid card, the motherboard or the ram?

what tests can i do to check montherboard stability?

thanks and i hope you can help me out

2 years old with 256 mb of system ram doesn't sound right