Howdy sorry, hope this is not too late.
With DDR2 4gb is the sweet spot, without much details on your current set up I suggest buy ddr2 2x2gb modules 800mhz for about £75
Make sure you put them in the correct 2 channels and that is better than you have.
Your bottle neck may be memory at the moment with 2gig
Also the 1TB western digital caviar disc will speed things up
So will a non-stock cpu cooler and a small over clock (tho' I dont know what your processor is)
So will a better graphics card - memory and graphics card at the moment are probably the areas that make the difference
That said,
With DDR3 6gb is the best
Increase these memory totals as much as you like but there is diminishing returns especially with games which are not generally multi-thread
You won't make much difference making a leap from 4gb to 8gb and I understand you will need Widnows 7 64bit to utilise more than 4gb anyhow
At this stage a decent dual or quad core
DDR2 800mhz 4gb
and a current graphics card is going to ace any game you care to throw at it
Once again the Ars boys have good solutions and read around on the tinterweb
http://arstechnica.com/hardware/news/20 ... -guide.arshttp://arstechnica.com/hardware/guides/ ... dition.arsKeeping what you got - may be 4gb plus a new graphics card is the solution, an after market cpu cooler and a mild overclock
Alternatively, go for 6gb ddr3 an amd cpu 550 - with a great gfx card - if you game this will ace everything for a long long time
i5 and i7 look great and are truly future proof but is costs a lot to be a pioneer and the system above or similar will ace COD and BF etc for a long long time
Post your current system so I have more details and I would love to offer an opinion as a rookie builder

I am sure there are folk here that can chip in a more expert opinion,
regards pod