Do not waste your time on XP. Use Vista or Win 7 64 bit. Most if not all newer games are optimized for Vista/Win7. Vista/Win7 also has better multimedia support. 4 gigs of ram is on the weak side, I'd go with 8 gigs.
As far as graphics cards go you say you want to play the latest and greatest. Question, what is the native resolution of the monitor you are going to use? If its 1920x1080 you will need way more graphics horsepower than a 5770 will supply. A 5850 or better would be needed. If your monitors native resolution is 1680x1050 or lower or you are willing to play games at lower resolutions than native a 5770 will get you by. LCD's tend to look soft when not running native resolutions so for best image quality its best to run native resolution.
If money is tight I'd get a 4870 1 gig instead of a 5770 because its faster for about the same cost. The only down side is no DX11 support. Not a big deal at this time. Even if you decide to build an Intel system I'd go with a ATI video card because Nvidia has nothing comparable to the 5xxx series cards at this time. If you are going to build an Intel system do not settle for anything less than a i5/i7.
My take on what you will need for a well balanced gaming system without breaking the bank. Having 8 gigs of ram instead of 4 will add approx $100 US dollars to the price. Going from a 5770 to a 5850 will add will add approx $150 US dollars to the price. Since the amount of ram and the graphics card are much more important than the motherboard and even the CPU to some extent if cost is an issue go with something like this.
ASRock M3A770DE Motherboard $60 US (save $100 US)
Phenom II X3 720 $125 US (save $40 US)
Neither of the above two changes should noticeably affect gaming performance. Very few games can even fully use a dual core CPU yet a quad core. The MSI 790FX-GD70 and Asus M4A79 Deluxe mainboards are nice but IMO overkill. Your not going to be running Crossfire or extreme overclocking are you?
Main Box*i7 930*GA X58A-UD3R R2*3x4 gig Patriot DDR3 1600EL*EVGA GTX 460 1 gig*Intel X25-M G2 80 gig*WD Green WD20EARS 2 TB*ASUS DRW-24B3LT*Samsung SH-S223L*Corsair AX750 PSU*Rosewill Challenger case*Windows 7 Ultimate 64 bit SP1