if your only at 1368x768 res then your 5750 shouldnt be too problematic. i game at the same res with a GTX 460 768MB and a phenom II X4 955, and can max out every game i have, the most demanding of them being crysis with a custom config, which i get ~40FPS
if you see the comparison your graphics card is quite a bit worse than mine, but the processor will certainly make up some of that difference, so you should be able to get decent settings on most games with the 5750
http://www.anandtech.com/bench/Product/173?vs=156
http://www.anandtech.com/bench/Product/88?vs=288
spend the money on the i5 2500k, a good motherboard with plenty of upgrade possibilities and whatever RAM you may need. also, dont worry about the thermal paste, the standard cooler comes with some pre-applied
*edit*
this is the ideal upgrade (but quite far over budget), as the motherboard supports crossfire/SLI should you want to add a second graphics card in the future, and it also supports all the Z68 motherboard features, one of which is lucid virtu, which saves a heap of cash on the electric bill
YOUR BASKET
1 x
Intel Core i5-2500K 3.30GHz (Sandybridge) Socket LGA1155 Processor - Retail with FREE TrackMania 2 Canyon PC Game £173.99
1 x
Gigabyte Z68XP-UD3 Intel Z68 (Socket 1155) DDR3 Motherboard ** LOWEST UK PRICE ** £99.98
1 x
Kingston HyperX Blu 4GB (2x2GB) DDR3 PC3-12800C9 1600MHz Dual Channel Kit (KHX1600C9AD3B1K2/4G) £23.99
Total : £297.96 (includes shipping: FREE).