Is a CPU upgrade a near-future possibility? If so, spend no more than the GTS 450 w/ MIR, and document the crap out of the rebate stuff.
If the price difference can be put towards CPU in the coming months, even on the budget end, that (saving nearly $65) could get you from affording an Athlon II, to overclocking an i3, or if a Phenom II or i3 is otherwise affordable, to a decent i5. The games you mention, and any similar to them, will need more oomph from the CPU than GPU.
OTOH, if that's too far out to worry about, then I guess get a card that's a tad overkill (given GPU usage on my GTX 460 1GB, w/ a C2D 3.2, I'm guessing 50% is actually being a bit optimistic, even w/ AA & AF).