lets see, the GTS 250 is equal to the ATI 4850 in performance, so that card should hold you down fine at a resolution of 1680 X 1050 or lower.
As far as overclocking the E4300, you may be able to reach 2.9 - 3.1 with a good aftermarket heatsink/fan. I had my E4500 @ 3.3 from stock 2.2 with a xigmatek HDT s1283.
questions i have are what resolution do you play at? (size of monitor) and do you already have this system like Enzo asked?