There is an interesting article here (http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2149&p=1) that talks about the association between a graphics card and CPU and whether or not buying that 'top of the range' graphics card will give you any better performance. Depending on the performance of your own system you may find that the investment in the spinky new and expensive graphics card provides you with zero returns.
Check it out!
TL.
There is a link on the first page to their previous article regarding how the various graphics cards scale on a given processor (admittedly a high end one) and you should read that in conjunction to get the full picture.
In my case with a 2.53GHz P4 then I will double or nearly triple my frame rate with a 6800GT - I couldn't do that with a system upgrade and keeping the existin card (FX5900). However, in a year or so I can upgrade board, CPU and memory to give another huge boost in performance because the GPU scales real well with processor power.
TLs point is valid! You need to look carefully at what you have in order to make the best of your upgrade. Nice article! Cheers TL