It's been a while since I indulged in pricing up a nice little computer that should play SL very well, so here we go again. I'm being a bit extravagant this time, I must admit.
Ground rules: we're upgrading, so we aren't getting monitor(s), a keyboard, mouse/trackball, or speakers. We already have those. Prices are rounded to the nearest dollar. You may be able to find these items for less, and you will be able to eventually, technology being what it is.
AMD Phenom II X2 550 Black Edition ($103 at newegg.com)
It runs at 3.1 GHz, with a generous 6 MB of L3 cache, and is rated at consuming just 80W. Dual core... but there are reports of being able to significantly overclock it and unlock the other two cores. I don't indulge in that pastime normally, but for that kind of gain, I'd be willing to try. Retail, so it has a fan and heatsink, but if you set out to overclock and unlock, you're going to want a hard core fan/heatsink.
GIGABYTE GA-MA78GM-US2H ($75 at newegg.com)
AM3 capable, though admittedly it doesn't do DDR3. Still, it does the job, is well thought of by most reviewers, is well-equipped with I/O options, including eSATA and 1394, and it has the Micro-ATX form factor, which lets us use...
APEVIA X-QPACK2-BK/500 ($100 at newegg.com)
In the old days, peripheral interfaces were separate cards that plugged into the motherboard's bus. As technology has improved, more and more stuff has come onto the motherboard--serial and parallel ports, floppy and hard drive interfaces, ethernet, keyboard and mouse, sound, and now often graphics, so that one needn't have all those slots and the motherboard can be smaller... hence the tiny computers from Shuttle and others, and hence the Micro ATX form factor, which for me provides reasonable flexibility (no sole source custom power supplies and such, and you can swap out motherboards), and no more insanely heavy cases! I wish they'd sell it with a modular power supply, to allow better airflow, but we'll stick with the 500W power supply that comes with it, so we choose...
XFX GS250XZDFC GeForce GTS 250 1GB ($150 at newegg.com)
It's a good card, and the GTS 260s want enough 12V current that the power supply that comes with the case would not suffice. We're not so hard core as to want SLI.
Western Digital Caviar Black WD1001FALS ($100 at newegg.com)
A 1 TB, SATA 3 Gb/s hard drive, 7200 RPM and 32 MB of cache. Remember when IDE hard drive prices dropped to a dollar per gigabyte, and we were all so excited? (Never mind that they were tiny, slow, and had pitiful amounts of cache by today's standards.) Now we're hitting, and sometimes under, the dime per gigabyte level for rotating hard drive space.
G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM ($50 at newegg.com)
Dual channel, of course.
SAMSUNG 22X DVD Burner Black SATA Model SH-S223F ($25 at newegg.com)
Blu-ray is still way too expensive for my tastes.
If you add the actual prices rather than the rounded ones, before shipping it comes to $598. Like I said, I splurged a bit.
Oh... software cost? $0, of course; we're using Linux. Someday I will back up all my data and experiment with LOTS of Linux distributions to see whether any stands out as snappier for Second Life use, and report on the results here.