Can my machine play oblivion/stalker, if not, upgrades?

I'm new to this site, it's great but i am intruiged by a couple of games. I wanted to know if my pc will play them well, if not then what should i upgrade?

Windows Vista Home
Intel P4 3Ghz
1.5 Gb RAM
ATI radeon sapphire x1900gt
19" widescreen monitor (just got it:D)

sounds good. anyone else? i hope i'm in for a treat with es4: oblivion, it LOOKS good.