U2 and UT2003 use just about the same engine.
In this day and age, if you're complaining about your computer being slow and you only have 256 megs of RAM, wake up. I was using 256 megs back in 1999, and it was good for a while. It ain't 1999 anymore, and RAM is dirt cheap. Don't run with less than 256. Take the "recommended" requirement for a game and double it--then you know you're peachy. In this case, I doubt it's your videocard, or your processor, or your motherboard. Only your RAM looks suspect.
UT2003 is highly limited by the CPU where U2 is not at all, Kiler. In fact, you cannot make the video card the bottleneck unless you A. use a custom map or B. have a really crappy videocard. Hell, even my GF3 wasn't the bottleneck to getting 50FPS until I set the texture detail to more than its 64 megs could handle.