Look at it this way
Look at it this way building a computer is like building a car for the ground up.
you need a good balance of cpu and gpu, I normaly look at it this way when I build someone a budget system I go about 10 - 15% above user needs, mid range I build at at 15 - 25% above user needs, high end work 25 - 50% and high end gaming is well 40%+ depending on customers budget as high-end gamers normaly want one of the top 3 -5 CPUs, RAM MoBos, GPUs, HDDs, Sound cards and ROM Drives there is.
You don't whay to get bottlenecked so realy do your homework you are not going to put and nVidia Asus Extreme 6800 Ultra on a Intel P4 S478 P4 2.40A GHZ because you found a Motherboard for the chip that has a PCI-E slot, just as you would not use and AMD FX55 with an ATi 9800 All In Wounder 128 MB card. It is best to do this; 1 Look at what you need and want out of the system, 2 Look at what you have to spend 3 Look at what you allready own HDDs, ROMS, RAM and so-on ask yourself are you going to still run this computer if not will it be worth the effert to incoperate these parts into the new system? 4 Look at building a new system in stages would be better. 5 (MOST IMPORTANT) Is building this system in my abilities?... Alot of people think they can build a system till they do it, building a system is not the world's hardest thing to do but it takes homework and some thinking. This is more so when you have a system and you are upgrading it because some newer parts will error or may cause problems for the system, BIOSs and Window's install configs can brake your brain. I/O settings alone even in the world of PnP can realy be a mind drain you in some cases. I had to flash a BIOS after putting a new sound card in a system one time after working one the system for two days... Tip if you have a problem with a new setup the problem is allways the most simple problem that will be the last thing you think of. I know I straid a bit there but it is all things you need to know and may-as-well get it all in one shot buy the way make sure you Power supply Unit (PSU) is up to the job if that is not them all the money you spent one the system will make it a expesive doorstop.
PS what games do you want to play and tasks are you going to run that is what you should start with look at the MSR for these games/apps and then go from there the othere thing is remeber ATi Cards and nVidia both make very good cards and fare around the same in AMD and Intel but nVidia will run better on AMD and ATi run alittle better on Intel when my website is up and running I will have a report on this posted compiled by students for the University Of Alberta's CEPD these kids know their stuff and have the toys to back their claims.