I'm on the same trip (wailing banshee power supply, odd shutdowns and ancient specs, coinciding with a tax refund!). Not sure what road yet, but here are the options I've dug up in the past few weeks.
Vista - I'm waiting until the clean-up crew clears the debris first, should take at least a year. Market seems to be flooded with Vista upgrade offers, makes me suspicious and some are offers in name only as they have to be redeemed now.
Dell - getting past the revulsion of actually contemplating buying one, the horror at de-crapifying the pre-installed software, the E521 looked like a good basic machine for $800-900 until tech support stated it only has a 305W power supply. XPS systems have at least a 375W but at $1K the barebones/configure it yourself units are on parity.
ABS.com - used them before for two desktops but they've become a gamers specialty shop. Nice systems and service but they don't offer for less than $1400.
PCClub - Folks here use them, for about $1100 on the specs below more options than the big guys (Dell, HP, Gateway) but not a lot of options compared to what follows
SysBuilder.com - No experience with them, site has too many options and a bit scant on information
CyberPowerInc - Again no experience with them, site has as many options at the previous but page layput out better for information
MagicMICRO - no experience again, site just as good as the previous two for options,but price seems to beat the previous two by about $100 less for same system layout.
The system spec I've been using to compare them:
CPU - Intel Core 2 Duo E6300 1.83GHz
MB with PCI Express slots and IEEE1394 connectors, & either SLI or crossfire ready
any case with at least 420W, in most instances as much as 500W or more for negligible difference
either liquid cooling or mucho fans
2GB DDR2
at least a GeForce 7600GT 256Mb PCI Express card
at least a 250 Gb SATA2 HD
16X-18X DVD burner
3.5 FD and an internal multi-type memory card reader
onboard sound
no monitor
MS XP Home/Professional/ with or without Vista coupon (and this is were it gets crazy)