you do not say what memory your graphics card is? But your system specs most users would think to be pretty good. If it aint broke etc. I have an xp3000 setup with only 512MB ram and 128MB graphics and that is pretty good.
you haven't really noticed any slowdown in performance with the latest games, and yet you're considering building a whole new PC for gaming purposes?!?! No offence intended, but it does seem a bit like a 'keeping up with the Jones' type job, rather than something you really need - I agree with howard60 - if it aint broke, don't fix it. Even if you spend £350 on a graphics card, there will still be somebody somewhere with a more powerful system :-)