Yeah as Originality pointed out graphics card brand isn't too much. Just pick the card that you like the best or have researched into (read: reviews). Keep in mind that noise is a factor; as toms hardware, for example have suggested that the Radeon 6990 (fastest graphics card) is loud as hell, whereas the slower GTX 590 is slower, but quiet as a mouse.
Deciding whether you're going to over-clock the CPU/GPU will be influential as well; you'll have to decide, then; whether you're going to settle for a stock heatsink or an aftermarket heatsink (capable of more efficient, better heat dissipation). Processor wise definitely go with an i5-2500k or i7-2600k they'll easily last you 5-10 years in my opinion.
RAM isn't much of a limiting factor these days, 4GB is fairly standard, but it's not unusual to have 8GB in a system, more than 8 though, is a waste of cash; unless you're doing something that benefits having all its program-instructions stored in RAM to speed up the operations and the time in which they run to completion. (Nothing an average user is really capable of).
Graphics wise: Again, totally depends on your resolution and whether you want to play games on higher resolutions at the most enthusiast settings; or you're comfortable running things at more modest settings like 1280x720, or 1440x900. You could easily spend nearly a 3rd of your budget on a graphics card alone. $2500 is a HUGE budget.
http://www.tomshardware.com/reviews/intel-...cking,2876.html
^ - This is what Tom's Hardware came up with for a $2000 budget performance PC.
(Benchmarks!)
Personally, I woudn't skimp on the motherboard since it's the piece that basically contributes to the overall foundation of the computer; it's the most important hardware component besides the power supply; and also provides the upgradability of your system.
To be honest, once you build your PC, I don't think you'll ever settle for a pre-made system again.