i3 2100 for £91 (i5 2500 for 150)
8gb ddr3 £38
you can allways upgrade the i3 to an i5 2100 or i7 2600 at a later date as its same socket (make sure motherboard supports though)
(you wont get much improvement sticking with core 2 ( i got a 3ghz core 2 duo e7600 and it can hardly run call of duty mw2, it runs at arround 20 fps
(if you mean boot times then defrag it and it will run quicker, or add an ssd and it will speed startup times but cpu,motherboard and ram are a better upgrade option first)
and gpu maxes out at arround 50% and cpu at 100%
It would be helpful to know more about your computer. Are you low on RAM? You could gain enough performance just increasing it. A dual core 2.6 ghz is usually adequate for gaming, just not amazing.
I agree with shanky that the best option for 2 cores is the i3-2100 with an H61 motherboard and 4-8gb of ram.
I use it for college and videogames. I use adobe cs5 for design work (the school part), and I just got Deus Ex HR and would like to play ES Skyrim. I was hoping just to get the videocard, but it turns out my cpu is not cutting the mustard