if you know how to build computers you need to check the prices sometimes its cheaper buy a brand name , the best is DEll for just cases and MB that it uses also some of the deals they have especially this month they are giving alot away like free printer and 17" or 19" LCD, The other companies are a pain to add hardware like harddrives, because you don't know whose Motherboard is used plus the cases are small and pain to take apart. You need to get the parts cheap to match brand name since you get OS for nothing.
I just brought from DELL different computers, I couldn't build them for the price I got them. I got was Dimension 8400 has Intel Motherboard with intel 915G chipset which has Matrix RAID onboard. I brought HomeXP,Intel made 775socket with Matrix RAID, intel540 cpu,512meg ram,80gb sata drive,48x cdrom, ATI X300 128meg PCIE video card NIC and Ultrasharp 17" LCD with 16ms refresh and Dell 720 Printer for $599. I plan to sell X300 and 80GB Sata drive and replace with either Nvidia 6800GT or ATI X800/850 video card and turn on the RAID and put in 2 @250gb sata drives, soundblaster Audigy2 Platinum,2@ DVD Dual Layer Burners all for less than $1000. the other was Windows Media Center 2005, Intel 560 CPU, 512meg ram, 160gb sata drive,soundblaster Audigy2, 256meg Nvidia 6800GT,16X DVDrom, 16X DVD Dual layer Burner,56K modem, NICand 3 year warranty for $725, I just missed a XPS generation 4 with intel560 2gb ram,audigy2,256meg Nvidia 6800GT PciE video, 16XDVD Dual layer burner,2@250GB Sata Drives for $1220. parts along cost $1500 without OS.