Okay to clear a few things up:
P55 and P67 series chipsets (not X58) are dual-channel. Meaning, using 2 sticks of RAM gives you double the memory bandwidth. This is why it is REALLY recommended to use 2 x 2GB RAM sticks unless you already have a 4GB memory stick with good speeds lying around (which I doubt). You need DDR3 RAM of course, with a speed of 1066-1600. These 4GB kits are getting much cheaper than before. Buying brands is better than bog-standard memory since the brands tend to come with cooling, are more stable, are overclockable, and allow you to overclock the CPU higher.
Now the graphics. Radeon HD 6790, GTX 460, GTX 560Ti. These are the cards you should be aiming for with such a budget. I have a GTX 260 and that has no problem playing everything on highest with no AA @ 1920 x 1080 (except Crysis 1 and Metro 2033, of course). Do NOT, I repeat, do NOT, get a Radeon 5xxx series card, not with the 6 series offering better performance for lower prices and lower power consumption.
This will help you much than I ever can:
http://www.guru3d.com/article/vga-charts-spring-2011/
This article compares 52 recently released graphics cards in multiple games. Check it out.
By the way, overclocking a graphics card is probably the easiest thing ever. Just dial up speeds with programs such as RivaTuner, MSI Afterburner, etc... until you get glitches then back down, stress test for 1 hour, and you're good to go. A good stress test program is OCCT. I got a 20%-30% boost with my GTX 260 with overclocking. Now it approaches GTX 280 / 285 speeds. Good way to save a few bucks too.
Now about Core i3, i5, and i7. Core i3 is dual core only (with integrated graphics in the CPU). Core i5 is both dual core and quad core. Core i7 is purely Quad core / Hexa-core. Core i5 2400 is basically the same as Core i7 2500 / 2600 save for slight differences in clock speed, no hyperthreading, and no unlocked CPU multiplier (who cares?). Basically, it's a good way to save money by going Core i5 2500. Do NOT get P55 motherboards as they are old gen. Get P67 / H67 / Z68 boards. Core i3 / i5 / i7 CPUs with 3 digits (Core i3 530, Core i3 540, Core i5 655K, Core i5 750, Core i5 760, Core i7 860, Core i7 870, Core i7 875K, Core i7 920, Core i7 930, Core i7 940, Core i7 950, Core i7 960, Core i7 965X, Core i7 970, Core i7 975X, Core i7 980X, Core i7 990X) are ALL old gen (Bloomfield & Gulftown). Get the Core i3 / i5 / i7 2xxx series. They are the new gen (Sandy Bridge).
If you have any questions, ask me by email. I can guide you through.
