Budget upgrade to improve framerates in modded games

I'm looking to start the new year by getting back to a game I've always enjoyed, TESIV: Oblivion (yea, I prefer it to Skyrim). Unfortunately, the last time I tried to revamp it via heavy, heavy modding... my framerate started getting shot to hell. This time, though, I wanna dive back in prepared... and with holiday sales everywhere, I figure there's no better time to toss in a new GPU or something and pray for perfection.

Problem is, my rig is 3 years old.... and I haven't really been following the parts market the way I should be. To be sure, I've read the articles Tom's has available (i.e. the December Bang for the Buck GPU list)... but, honestly, when it comes to running 150-odd mods and several major visual overhauls, I'm just not quite sure whether I should focus on improving the GPU, memory, or CPU/Motherboard. Even if my guess (GPU) is right, I still have no idea whether I need a recent GPU+Less VRAM or an older GPU+More VRAM. So, I've come to the right place for help.

Summing up, I need a way to massively improve my framerate in older, heavily modded games like Oblivion. I'm aiming for a ~$100 upgrade, but if that isn't feasible I'm willing to entertain pricier options (just so I know what I need to budget for).

Here's my PC's relevant specs:

i5-750@2.66ghz
700W CoolerMaster (so power shouldn't be an issue)
http://www.newegg.com/Product/Product.aspx?Item=N82E168... <-link to GPU, I get anywhere from 12-30 fps with this (in TESIV, modded).
2x2gb DDR3 1600 (low, I know... that's why I'm considering a memory upgrade instead of a new GPU)
1280x1024 Monitor (Yup, I love that ratio and I refuse to leave it)

EDIT: Just thought I should add that I don't have a GPU brand preference... I grabbed AMD for the Bang/Buck, but I've heard Nvdia does better with Oblivion.

Thanks for your time, and I'd appreciate any help.