if i remeber correctly the e6300 can be overclocked pretty well. 500mhz isnt to hard and 4ghz isnt impossible. that will put you in playable gaming area... next get another gig of ram which should be exactly the same as the ram you have in both timings and latencies if you cant get exactly the same make and model. next a gfx card. you want a minimum of a 96gt but a gts250 would be better.
all this will need a minimum of a 450w psu to run it...
to upgrade what you have will cost about £150 and will give you an entry level gaming pc that will play everything you want up to 2011... some of todays games need win 7 and a quad core... which is likely to be out of your budget.
but hey if you can stretch to £500+ you can get quite the decent i5 system...