right now is a great market for computers. you can get some seriously awesome hardware for a great price. i've seen multiple PC gaming sites build great PC's for really low prices, and some of those PC's were built for as low as $400 (USD).
CPU's haven't been really hot so far this year other than a couple of new hex-core CPU's from both AMD and Intel. as it has trended for the past couple of years, Intel is more powerful while AMD is more affordable.
GPU's, on the other hand, have been really hot this year (no pun intended). AMD has expanded on their DX11-based Radeon 5000 series to round out their portfolio while Nvidia has been playing catch-up with their GTX 400 series. Nvidia sports the fastest hardware, but its not that much of an advantage. think of it more as the difference between the GeForce 7000 series and the Radeon X1000 series. its pretty neck and neck across the board in terms of pricing and performance. the major difference is that Nvidia is still rolling out their new series while AMD is set to launch the new Radeon 6000's in time for the holidays.
otherwise, the landscape hasn't changed much, and there's a ton of value to be had if you know where to look. on the CPU front, AMD has some excellent quad-core CPUs with the PhenomII series. personally, i would go for the Phenom II 955 Black Edition. it has an unlocked CPU multiplier which makes overclocking a breeze. if you do go Intel, though, you can't go wrong with a Core i5. performance is on par with the PII 955, and they overclock extremely well. the difference is that they don't have an unlocked multiplier, and that makes things more difficult when overclocking.
with motherboards, i would recommend looking for a board that supports the new SATA 6.0 GB/s standard. it'll give you more bandwidth for whatever storage medium you choose (its backward compatible with the older SATA 3.0GB standard). if you do go Intel, you can also look for boards with the new USB 3.0 tech. AMD, however, won't have USB 3.0 until their new lineup of CPU's launches (hopefully) later this year.
on the GPU side, i would recommend the GeForce 460, Radeon 5830, or the Radeon 5850. all of them are excellent performers at a great price point. the performance edge goes to the GeForce.
as far as RAM is concerned, that will depend on which CPU you go with. if you go AMD, you'll want a 2x 4GB DDR3 kit(8GB total) for the duel channel setup. if you go Intel, you'll want 3x 2GB DDR3 kit (6GB total) for the triple channel setup (which AMD doesn't currently support).
otherwise, you can get some great deals on hard drives. hard drives are cheap, faster than ever, and 1 or 2 TB is a ludicrous amount of storage for most people. i personally recommend the 1TB Western Digital Black. i use it in my current PC, and its the fastest drive i've ever had.
of course, somebody is bound to mention Solid State Drives (SSD's). those drives are wicked fast, and they will allow Windows to boot insanely fast. however, they're also ludicrously expensive in terms of cost/GB. taking a quick look over at Newegg, you can get a 32GB SSD for ~$70 USD. doesn't sound like much until you consider that you can get a 750GB HD for the same price. just keep that in mind if you do go that route.
as for everything else, not much has changed. if you use the hardware specified, you'll probably need a 650W PSU. if you want Blu-ray, its available, and its not all that expensive, either. if you have money to spare (its easy enough nowadays), i'd put it into some better cooling in the form of a CPU fan/heatsink and perhaps a couple of high CFM fans so that you can get some better overclocking results.
anyways, good luck with the build, and keep us updated on how its going.