Well, for the latest "CPU Crushing" games, there are only two CPU's I would suggest in order of preference. The Intel Core 2 Duo (2.4-2.6GHz or higher). The AMD Athlon64 X2 4800+ or the Athlon FX60. Next is the RAM, it doesnt matter what RAM you get as long as it matches what your motherboard can accept, what matters is how much. Try and get between 1 and 2GB.
Last then, and very important, is the motherboard. All motherboards you will get now will be SATA boards, SATA is a new type of interface for connecting Hard Drives and the like. These will have the older PATA connectors for your older Hard Drive, but if you have some cash left over, you could always get a decently sized SATA drive and keep the older one to run windows on and install all your games and the like to the larger SATA drive.
I couldnt say what motherboard to get, but if you go to Komplett.ie, click Mainboards (under componants) and click AMD 939 for the Athlon chip or Intel 775 for the C2D. This site, while listing everything in €'s will give you a good selection of boards with the full details for each one, and what processors and RAM it can take. Pick one you like and google it, google will show other sites that sell it and you may be able to find a site that sells them in your area.
Oh, and then there is the graphics cards, the one you chose is a good card, I have the very one in my Macbook Pro
There is one problem though. I assume your PC has Windows, and I assume it came with your PC, legally (this is a grey area) you cannot use the same copy of windows on your new PC so you will have to buy a new copy. If you have XP, then you will have no choice because it wont activate on your new machine once you have replaced the MB. You will probably have to buy a new copy, however, if you bought a retail version of Windows, you can install away on the new machine with no problems. The other problem is that unless you built this PC yourself (say if you got a Dell) you may not be able to install the new componants into the case, I know for a fact that Dell cases are specific to Dell motherboards and no other board will fit into them. The same is true for Compaq, HP and a few others. All said and done, you may end up having to buy a new case, but your old drives and the like will fit into that without hassle.
If you can tell me more about your old system, I can advise you better.