I picked up a copy of Doom 3 and played a little bit myself. Needless to say, I HAVE to upgrade my computer ASAP. I played at 640x480 on medium settings and things were still a little bit choppy. That being said, however...
Sure, it's a simple FPS where you blow everything away in sight. But unlike a lot of other FPS that I've played, Doom 3 was CONSTANTLY keeping me on edge. I couldn't believe how much I was screaming like a little girl whenever the newest zombie/monster/whatever would show up. I hadn't been this scared since I played the original Doom over 10 years ago.
Considering that an upgrade will only make things better, oh boy...
Anyway, regardless of whether you get it now or get it later, this is a must-buy. It's that good.