It's strange that you get half FPS in C-101 vis-a-vis MiG-21. Were you using the interior lights?
Btw you can upgrade that GTX 275 pretty cheaply now, for instance a GTX 750ti can be had for like 130 USD.
Gtx 275 compared to GTX 480 (big difference) http://www.tomshardware.com/charts/2010-gaming-graphics-charts-high-quality/compare,2463.html?prod%5B4504%5D=on&prod%5B4524%5D=on
And GTX 480 compared to GTX 750 Ti (750ti around 90-100% performance of GTX 480) http://www.anandtech.com/bench/product/1130?vs=1135