I have $400, upgrades to play oblivion?

I have about $350-400 to upgfrade my pc, with the likes of playing oblivion, vegas and maybe fear on meduim. My PC isn't all that strong, so if you could upgrade it with $400, what would you choose and would the end result play es4 oblivion nicely? And maybe some newer games on low? BTW, I'm playing at 1280X1024 Resolution.

Here she is:  CPU: P4 631 3Ghz , 512mb pc4200 RAM w/ 1 slot available, ati xpress 200 128mb garbage onboard video
Windows Vista home basic, I have one PCI-E X16 slot.

I was thinking of buying a Gig of corsair valueram, $52, and a 8600GTS @ $200. Then an enermax 485W noisetaker II PSU for $90. with shipping this is $370. What do you think? Would my "after" specs play oblivion? What settings?

Why? I thought I told you everything...

Id run dxdiag, but i'm at my buddies house right now. Don't remember.

That's a tough decision. 2 GB Ram is very important, but so is the video card :-S (which needs a better psu.) Does your mobo support Core 2 Duo? If so, I would get:

An e4300 which is supposed to be $110?

2 GB PC 4200 RAM (G.SKILL) $105

Ultra X-Finity 600W PSU $70

That comes up to $285. That leaves $115. If you save up a little more you can get an 8600gt.

If you absolutely need it under $400, get 512MB Patriot RAM ($34) instead of the 2 GB. This would bring the total to $214, giving you $186 to get the 8600gt.

I think 2 GB of RAM is very important though, as is the processor. A pentium 4 will be a huge bottleneck on an 8600gt or 8600gts, so it's not that great of a deal to not upgrade the videocard w/o the cpu.

Also, the PSU isn't a great brand, but with this budget, your down to pretty slim pickings. The specs are more than enough to power all of this stuff.