Here's a system that I've had planned out for a build for a few months now. Back when I first thought of it I'd planned it to be pretty top-o-the-line build for gaming but I haven't followed through with my plans due to $$. If you're serious about it though I'd love to see someone put it together or at least use it for a starting point for something else.

Case - Thermaltake Armor A60 Gaming Case - $74.99
I chose this because it supports USB 3.0, comes with blue fans and comes from a great company.
MoBo - P8P67 LE-R3 LGA 1155 P67 ATX - $134.99
This also supports USB 3.0 and goes nicely with the CPU and RAM that I've chosen.
CPU - Intel Core i5 2500k - $179.99
(Unlocked Edition for even easier overclocking, and it can reach a Turbo Boost speed of 3.7Ghz all by itself, plus I've heard of them topping 4.0Ghz on air cooling.
RAM - G.Skill Ripjaws X Series 4GB (2x2GB) DDR3 1333 - $31.99
Your basic DDR3 1333 RAM with low CAS numbers for faster performance.
GPU - Sapphire 5770 Vapor-X Edition - $99.99 After Rebate
I run one of these in my current gaming system and I absolutely love it. It runs really nice and will outperform some of the 6xxx series cards when overclocked , plus it lights up blue which would go well with the blue fans in the case and in the PSU.
HHD - Western Digital Caviar Black 750 GB - $124.99
Nice HD with 750GB of storage and SATA III connection, 6Gb/s transfer rates and a 64MB cache.
PSU - Corsair Gaming Series GS600 - $94.99
I chose this cause it has the option to light up 3 different ways and coming from Corsair it's gotta be good.
Optical Drive - LightScribe 24x Dual Layer Reader/Burner - $24.99
OS - Windows 7 Home Premium 64-Bit OEM - $99.99
Personally I think the upgrade to 7 Ultimate is a waste of $$ so that's why I chose Home Premium.

So that's what $866.91 would get you. Not sure what kind of budget you had in mind (and that's really where you should start) but that's what I had figured up for my next system. Keep in mine though that doesn't include any peripherals like mouse, keyboard or mouse. I hadn't factored these in though because I was planning on using my existing ones.
Anyways, that's what I had figured up. Enjoy.