MSI MBOX K8MM-V AMD Socket 754 AMD Athlon64/Sempron VIA K8M800 2x 184Pin VIA UniChrome Pro 3D Graphic Barebone
Specifications

Model
Brand MSI
Series MBOX
Model K8MM-V
CPU
CPU Type AMD Athlon64/Sempron
CPU Socket AMD Socket 754
FSB 800MHz HyperTransport
Chipset
North Bridge VIA K8M800
South Bridge VT8237
Memory
Memory slot 2x 184Pin
Memory Type Supported DDR333/400
Max Memory Supported 2GB
Expansion Slots
AGP AGP 8X
PCI Express None
PCI 3
Storage
IDE ATA 2x ATA 100
Serial ATA 2x SATA 150
SATA RAID RAID 0/1
Graphics
Onboard Video VIA UniChrome Pro 3D Graphic
Video Memory Shared 64MB
Audio
Onboard Audio VIA VT1617A
Channel 6-CH
Communications
First LAN VT6103 10/100Mbps
Max LAN Speed 10/100Mbps
Extension Bays
3.5" Internal bays 4
3.5" External bays 2
5.25" External bays 4
Front Panel Ports
Front USB 2
Front Audio Ports 2 jacks
Back Panel Ports
PS/2 2
COM 1
LPT 1
VGA 1
Rear USB 4
RJ45 1
Rear Audio Ports 3 jacks
Power Supply
Power Supply 300W

I love AMD products but at the minimum try to get one that has a socket 939 for the processor. The advantage of that is if you want to upgrade your processor your options will be much better. You'll be able to put in a nice dual-core processor in a 939 socket. Also you want to get one that has a pci-express slot for the video card. AGP cards are just about dead.
This is just a suggestion, if i was buying a barebones system i like this one at Tiger Direct I've ordered from them before so i think they're a pretty reputable company. I hate rebates though. The advantage of this one is the only thing you'll need is the operating system. Also the motherboard is currently one of the best on the market and it supports AMDs current top of the line processor. I don't know if you want to spend a lot of money when you build your first computer because if you mess up that's just more money wasted. What i did first was take an old one apart and put it in a new case. Once i did that it gave me the confidence to build one.