1280x1024 is low-end by current gaming hardware standards. That system will run just about any game at max settings with ridiculously high AA/AF settings and still probably break triple digit FPS. For all but the most demanding games (Crysis: Warhead, Far Cry 2, etc.), you should get good performance at least up to 1680x1050 with everything maxed and most games should run well at 1920x1200 with AA/AF toned down a little.
If you want to build a powerful machine for $1000 that's relatively future-proofed, what you have is solid aside from the triple channel ram issue and the advice here will make it better within your budget. I would stick with a single 4870 for now - as I said, that alone will perform far better than what you say you are looking for, and you can buy a second one in a year or two (should be dirt cheap by then) if you need more performance but don't want to splurge on a newer card.
If you want to tone down your budget and make a machine that will kick butt at 1280x1024 for quite some time with room to upgrade later, you could get a much cheaper AMD system. The i5 is an amazing chip, but for gaming it's totally overkill (you will be limited by your GPU speed with a chip even half as powerful as the i5, so that extra power mostly goes to waste in gaming). You can get a Phenom II x3 720 Black Edition, which will perform almost as well in all but the most extreme multi-threaded processes like video rendering, for $120 US off newegg. It has an unlocked multiplier, which means it is extremely easy to overclock (Intel only does this on their uber expensive Extreme Edition chips), and you can save money on the motherboard, too - a quality 770 board with DDR3 can be had for around $80 US. That's right, you could have a CPU and motherboard for the cost of just the CPU with the i5 platform! That's $140 or so you can keep, or put toward components that will actually make a difference like GPU or investing in a RAID.
Speaking of RAID... For just the cost of a second hard drive, assuming your mobo supports it (most do), you could have an entry-level RAID setup and get much better performance. Aside from a better GPU, it's one of the easiest ways to tangibly boost gaming performance (at least load times), as hard drives are among the slowest components still in use in modern PCs and bottleneck pretty much any process that uses them.