It's been a while since I sat down and priced a computer for running Second Life. It's kind of fun, save for the part where I don't get to spend the money to actually put it together. :)
Case: I like not needing a huge heavy case, so it's Micro ATX for me. The Apevia Q-Pack 2 has been my choice for some time, but the package includes a power supply that isn't what one would really like: it's not modular, so you have to deal with all those extra cables, and these days graphics cards want more extra power connectors than it comes with. So, this time I pick a somewhat larger Micro ATX case that doesn't come with a power supply: the Compucase HEC 6T Series 6T18BB, $45 at newegg. (Yesterday it was a "shell shocker" and going for $25.) The Antec Neopower 650 is modular and not outrageously expensive, at $80.
Motherboard: the GIGABYTE GA-785GMT-USB3 is a step up from the motherboard I currently use, and supports USB 3. $95.
CPU: One of the Phenom six cores is awfully tempting at $200, but SL doesn't take very much advantage of multiple cores, at least not yet. The Phenom II X4 945 is just $140... or it may be better overall to get one of the dual or triple core CPUs, or a Propus quad core if you are willing to forsake the L3 cache, for about $100 and spend about $100 on an SSD to put the operating system and Second Life and its data on.
G.SKILL Ripjaws Series 4 GB (2 x 2GB) is of a speed the motherboard supports and is about $100 at newegg. Watching the RAM usage when I fire up Kirsten's client makes me wonder whether 4 GB is enough, but maybe that's a memory leak.
The nVidia 460 is getting rave reviews for price/performance. The GIGABYTE GV-N460OC-1GI GeForce GTX 460 1 GB is going for $230.
We'll get a hard drive that's "just" 750 GB: Western Digital Caviar Black WD7501AALS, $80.
DVD/CD burners are pretty much commodity items these days: ASUS DRW-24B1ST/BLK/B/AS SATA DVD/CD goes for $24.
Not counting shipping, if you pick the six-core it comes to $854. We didn't go for cheap this time around.