Get more RAM (as much as you can afford/fit after CPU + Video Card)
Get a new CPU (4 core minimum, I recommend the Phenom 965s or 1055s, they're quite good for their price points)
Get a new graphics card ($200-250 range one should be good enough)
RE Ken: There's a few things we can be certain of:
You can never have too much RAM
a two core CPU will be brought to its knees by this game if current titles out right now are any indication of what's to come in the next year
a GTS 250 is getting dated
From what I've read, most games don't even fully utilize 2 cores, I doubt 4 cores are going to be much of an improvement. Nevermind Bethesda's crappy track record for taking advantage of more than one core. Remember, many people buy pre-assemebled PCs, which tend to have run-of-the-mill hardware - probably not quadcore.
@the OP: I'm more familiar with AMD CPUs, so I don't really know how the Intel/other brands stack up. Don't bother with quadcore unless you find one on sale; keep in mind most games will mostly use one core, and probably only use a second core for the physics and some other effects. It is highly doubtful the developers are going to bother writing code and tweaking the engine to utilize more than 2 cores (though it would be nice if the devs did do that). The previous TES game engine tended to be a CPU hog, but I don't know if the new engine will be that demanding on the CPU. You might need a GPU upgrade down the line, but my hunch is that your set up will meet at least the minimum requirements.
EDIT: I have windows XP 64-bit with 3BG RAM. I think the developers have to code the engine to take advantage of oodles of RAM; iirc, some games put a hardcap on RAM utilization, and will only use, say, up to 2GB no matter how many gigs you have stuffed in there.
EDIT: Disclaimer: I'm not a computer guru or authority or whatever, so some of my info may be outdated or flawed.