My favorite place for designing a computer is www.newegg.com which has some really good deals if you know what you're looking for. Chances are the links I'll post, and the price of the item itself will not stay the same for long. This is just an example of a machine I'd recommend building as of the above posted date of May 22nd, 2008.
MSI MBOX K9N6SGM-V AMD Socket AM2 NVIDIA GeForce 6100 /nForce 405 chipset 2 x 240Pin GeForce 6100 Barebone - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16856167009
Listed at $104.99
AMD Athlon 64 X2 4400+ Brisbane 2.3GHz 2 x 512KB L2 Cache Socket AM2 65W Dual-Core Processor - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103213
Listed at $64.99
A-DATA 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16820211188
Listed at $81.99
Western Digital Caviar SE WD1600AAJS 160GB 7200 RPM 8MB Cache SATA 3.0Gb/s Hard Drive - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16822136075
Listed at $44.99
Pioneer Black 20X DVD+R 8X DVD+RW 10X DVD+R DL 20X DVD-R 6X DVD-RW 16X DVD-ROM 40X CD-R 32X CD-RW 40X CD-ROM 2MB Cache IDE 20X DVD±R DVD Burner - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16827129018
Listed at $29.99
Baseline computer itself subtotal at $326.95 that is without shipping configured in, no monitor, keyboard, mouse, Operating System, and no video card(the motherboard has an onboard video card). If you want to figure in shipping then to give you a rough estimate, to me it would cost $24.04 which brought the Grand Total up to $350.99
If your intentions is to upgrade and already have a monitor, keyboard, mouse, and operating system disc then this is a cheap and powerful upgrade. If you want or need all the pieces then continue on.
ASUS EN8400GS SILENT/HTP/256M GeForce 8400 GS 256MB 64-bit GDDR2 PCI Express x16 HDCP Ready Video Card - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121240
Listed at $34.99
Hanns·G HW-173DBB Black 17" 8ms Widescreen LCD Monitor 250 cd/m2 500:1 - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16824254018
Listed at $149.99
LITE-ON SK-1788/BS 2-Tone 104 Normal Keys PS/2 Wired Standard Keyboard - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16823107120
Listed at $6.99
Logitech SBF-90 Black 3 Buttons 1 x Wheel PS/2 Wired Optical Mouse - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16826104051
Listed at $5.25
LOGISYS Computer SP6000-SL 32Watts 2.1 Soho Multimedia Speaker System - Retail
http://www.newegg.com/Product/Product.aspx?Item=N82E16836157001
Listed at $23.99
Microsoft Windows XP Professional With SP2C - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E16832116400
Listed at $129.99
Everything on this page subtotals to $678.15 which is pretty good for a complete/custom above average build computer. Something to note is that the motherboard's PCI Express 16x slot only run's in 8x mode. This is why I chose one of the cheaper video cards. It'll still play games but it won't play some of the newer games that have pretty high requirements.
On a side note, this kind of design is what I'd personally like to get for dual booting of both Windows and Linux. Would work great for programming under linux as it'd be so fast for compiling code and during times of boredom could reboot into Windows to play a PC game.