Yeah, you gotta check the mother board site, and make sure every part you've chosen out is compatible. You also have to make sure the mother board is compatible with the case too.
Here's a relatively cheap config (links and all), lacking keyboard/mouse/monitor/speakers, that I'm pretty sure is compatible with every other part in it:
Mother Board:
ASRock "775I65GV" i865GV Chipset Motherboard For Intel LGA 775 CPU -RETAIL - $59
http://www.newegg.com/app/ViewProductDesc.asp?description=13-157-036&depa=0
Processor:
Intel LGA 775 Pentium 4 520 2.8 GHz, 800MHz FSB, 1MB L2 Cache, Hyper Threading Technology - Retail - $158
http://www.newegg.com/app/ViewProductDesc.asp?description=19-116-185&depa=1
Ram:
Kingman 512MB DIMM DDR Ram - $60
http://www.newegg.com/app/ViewProductDesc.asp?description=20-156-006&depa=1
Hard Drive:
Hitachi 80GB 7200RPM SATA Hard Drive, Model HDS722580VLSA80, OEM - $59
http://www.newegg.com/app/ViewProductDesc.asp?description=22-145-049&depa=1
Video Card:
eVGA NVIDIA GeForce FX 5200 128MB - $45
http://www.newegg.com/app/ViewProductDesc.asp?description=14-130-210&depa=1
Sound Card:
Sabrent 6-Channel 5.1 Surround Sound 3D PCI Sound Card Model SBT-SP6C - Retail - $11
http://www.newegg.com/app/ViewProductDesc.asp?description=29-130-001&depa=0
CD/DVD Drive:
Rosewill 52x32x52x16 CD-RW & DVD Combo Drive, Model C523216, Retail - $30
http://www.newegg.com/app/ViewProductDesc.asp?description=27-190-002&depa=1
Floppy Drive:
Samsung Black 1.44MB 3.5inch Floppy Disk Drive, Model SFD321B/LBL1, OEM Drive Only - $7
http://www.newegg.com/app/ViewProductDesc.asp?description=21-103-203&depa=0
Modem:
Innocom V.92/ 56Kbps Software PCI Data/ Fax Modem, Model IT-MI220 - Retail - $7
http://www.newegg.com/app/ViewProductDesc.asp?description=25-100-102&depa=0
Case:
JustPC Silver/DarkGray ATX Mid Tower Case With 450W PSU, Model "GA808ATXBLK" -RETAIL - $46
http://www.newegg.com/app/viewproductdesc.asp?description=11-200-023&DEPA=1
Maximum Price: $500
Total Price: $482
Price is around $482 last I checked the prices (a couple days ago, though newegg might have upped the prices since it's not winter holiday time anymore), not including shipping and handling. But it's still a pretty nice computer for cheap, though the video card might need upgrading.
Cheers,
Preston
Intel Celeron 1.3 Ghrz | 512 MB Ram | NVIDIA GeForce FX 5200 128MB
MSVC++ .Net 2003 | Wings3D | CharacterFX | Gimp v2.0
Prayne de crabug ahm rinedere be-yogt iglo kes gron