Here's a system that is reasonably priced and expandable.
http://cgi.ebay.com/INTEL-QUAD-CORE...145670QQihZ020QQcategoryZ140075QQcmdZViewItem
INTEL CORE 2 QUAD PROCESSORS Intel Core 2 Quad Processor Q6600 1066MHz 8MB LGA775 CPU
INTEL PROCESSOR COOLING FAN INTEL STANDARD PROCESSOR COOLING FAN
SOCKET 775 MOTHERBOARDS Asus P5B-VM LGA775 MATX Motherboard
DDR2 RAM DUAL CHANNEL 4GB (4x1GB) DDR-2 667MHZ PC-5400
SATA II HARD DRIVES 320GB SATA2 7200rpm 16MB CACHE (HIGH PERFORMANCE)
DVD-RW DRIVES 18X DVD-RW DUAL LAYER W/LIGHTSCRIBE
PCI-EXPRESS VIDEO CARDS 256MB CHROME VIDEO ACCELERATOR (ONBOARD)
SOUND CARDS 6-CHANNEL DIGITAL SOUND ONBOARD
NETWORK CARDS 10/100/1000 Fast Ethernet Network Card (onboard)
COMPUTER CASES BCC MID-TOWER ATX W/TEMP CONTROL LCD (BLACK)
POWER SUPPLIES OKIA A-POWER 600 WATT ATX POWER SUPPLY
KEYBOARDS 103 Keys PS2 Basic Keyboard (Black)
MICE I-MICRO 800dpi PS/2 Optical Mouse (Black)
SPEAKERS SUPER TALENT 2.0 Channel Multimedia Speaker System (Black)
Software : All driver disks and manuals are included.
DVD & CDRW software also included.
System comes ready to load any OS (Win98, Me, 2000, XP, Vista)
Operating System: Operating system is optional. If you add operating system, we will install and configure it for you.
Condition : All system components are brand new. System is completely assembled and tested.
Warranty 3-Year Limited Warranty w/LifeTime Free Technical Support
Shipping: SHIP BY UPS/DHL GROUND, 2ND DAY AIR AVAILABLE!!!
I own 2 of these and have bought systems from this guy for a while. A number of my friends have bought systems as well and we all agree these systems SCREAM!
Here's a link for an under $95 valid source for Windows XP Pro 64 bit:
http://www.von1.com/shopping/descriptions.php?auth=ArdqI8v4k4qFXtC4d314Xkh1dPToF5/4D9cb/+9iixs=
I prefer the 64 bit OS. I've got it loaded on all of my trading computers on these systems and have not had a single problem in 18 months. Finding drivers for multihead video cards is not a problem either. I'm not using Vista yet due to bugs and trading for a living needs to be bug free.