So I've priced out my best options - you can skip past all the parts to the prices, comparisons and conclusions if that sort of thing makes your eyes glaze over:
Dell Dimension 8400, w/3.6Ghz P4, 2GB 533Mhz DDR2 SDRAM, 250GB SATA hard drive, Creative SB Audigy 2 ZS sound card, 3 year onsite support, 2005FPW wide screen 20" flat panel, 16x DVD-ROM, 16x DVD-+RW dual layer and 256MB Radeon 850 XT graphics card after taxes, discounts and rebates, comes to about $3600.
Or, I can build a Scobleized (tm) PC with Ever Case tool-less case, Asus A8N-SLI motherboard, AMD Athlon 64 +4000 CPU, 2GB 400Mhz DDR SDRAM at 2.5-3-3-6 timings, 250GB Western Digital SATA hard drive with 8mb cache, Creative SB Audigy 2 ZS, 16x Asus DVD-ROM, 16x Plextor DVD-+RW dual layer, 256MB MSI Geforce 6800 Ultra graphics, Silenx iXtrema Pro 600W power supply, Silenx 120mm case fan, Silenx 92mm fan for CPU cooler, ThermalRight XP-90 CPU cooler and Zalman ZM-NB47J northbridge heatsink which after taxes and shipping all comes to $2222.80. So if I tack on $600 for Dell 2005FPW flat screen, comes to $2822.80.
With the Dell, I get next day on site support, very good tool-less clamshell case, slower CPU, lower wattage power supply (means no upgrade to Geforce Ultra 6800 - Nvidia recommends 550W PS or better for that card) and not quite as good graphics card.
With custom built, I get top notch motherboard, pretty much fastest CPU for games, awesome graphics card, very fast memory, heavy duty and silent power supply, more noise from the extra fan (dell has 1 case fan + PS fan, mine would have 2 fans plus PS fan), self support (only a bad thing if something fails, since I'm quite capable of fixing problems with Windows) and I get the fun (or stress as some may see it) of putting it all together.
So basically that 800 dollar difference comes down to the software you get from Dell, the labor in building it and the support. The difference in price is murkier than what it appears...the power supply in the custom built system is $200, plus those fans aren't cheap either, the graphics card on the Dell is $100 cheaper street than the Geforce 6800 Ultra, their motherboard isn't SLI, but their memory is more expensive (have no idea why, it's just standard 533Mhz DDR2 memory which can be had for as low as $200 for 2 x 1GB modules) by about $150. Factor in the $150 savings on the LCD panel and the $200 rebate and Dell is basically subsidizing the desktop and still coming out around $1000 more than what I would pay.
Given all that, the question is which system would benchmark best doing the games that I play and the other real world tasks that I do...then again the Dell better be a lot faster for that $1000.