Okay, a comment, because I feel like upgrading my PC too (it from Sep 2007).
Using a Single grafic card is better. Better prize/performance, better stability, better compatibility, better for cooling.
Using a single-GPU grafic card is also better, for the same reasons.
But starting with the CPU:
Intel Xeon E3-1230v2 - Quadcore with SMT, 3.3GHz base, 3.7GHz Turbo, 8MB L3-Cache, DDR3-1600 controller inbuild, 69W power usage, socket 1155, not good for overclocking because its a server cpu - 190€
or just little fast compared to price
Intel Core i7-3770(K) - Quadcore with SMT, 3.5GHz nase, 3.9GHz Turbo, 8MB L3-Cache, DDR3-1600 controller inbuild, 77W power usage (because of inbuild IPG), socket 1155, choose (K) for open multiplier for overclocking - 300€
RAM
as Boris wrote, most games can't handel 4GB (2.7GB with 32bit and 4GB with LAA-Patch). Only x64 games can use more. Simple calculation, take that 4GB and put the OS on top (900 for me with Win7prox64), so
6GB DDR3-1600 (no special manufacturer) should be enough for now - 40€
If you want to take it a little more into the future, take 8 or 12 GB, but buying that later when really needed isn't wrong.
grafic card
best prize/performance yet
AMD Radeon 6950 with 2GB vram - 175€
more up to date and fastest single gpu card available
AMD Radeon 7970 3GB (for example Gigabyte Windforce 3x) - 370€
or if you don't like AMD and want to go with Boris opinion about bad AMD drivers
NVidia GeForce GTX 670 2GB - 360€
or
NVidia GeForce GTX 680 2GB - 430€
Motherboard must fit the connectors you need and features you want, so no suggestion from me, but es you see above, both CPU use socket 1155. Take a MoBo with good overclocking features for the 3770K. Take a MoBo with at least some overclocking possibilities for the 3770. No need for overclocking features for the Xeon, as it nearly can't be overclocked. If you want to overclock anyway.
More important to prevent stuttering because of loading in background (streaming) and shortening loading times: take a SSD. One for games (64GB or more), one for the OS (windows) (64GB) - and perhaps one for the pagefile (swapfile)(1.5 to 3 times greater than your RAM, so 32GB should be enough). All other programs like word, exel, photoshop etc and all downloads, documents, photos, movies, small games or games you don't play anymore go onto a 2GB HDD.
And of course, you should use a x64 OS
That's one of the best setups you can get for now, at least when looking at price/performance and if you want to use it for about 4 years from now without (the need or the will) great upgrades.
And I won't take the i5-2500 anymore. Look at the chart at the middle ot
that german page.
You can choose between Anno2070, Battlefield3, Dirt3, Shogun2, Starcraft2 and Skyrim.
And you'll see, that the i5-2500 is starting to fall back behind the leading group, so it's a CPU for now, but not for the next 4 years. And all other CPUs, that are even better than the ones I mentioned above, are just too expensive and do not bring more performance or at least not noticable in games.
If you do rendering, video converting etc, well, _than_ you might want to have a 6core (with SMT) and more RAM. But only then. No need for it as Gamer. Not for now, not for the next 4 years. Not even with Crysis3 (even if it's x64 and a similar graphic leap to other games like Crysis1 was to other games).
So with the components I wrote down here, you'll spend only half the money but get the same gaming performance for the next years. After that time, you'll have to update your "dream PC" as well as mine
Just imho.
/edit: I want
these effects in Skyrim.
no!