I'm sure you could build a decent rig for under $600 with all parts from NewEgg.com but as others have said you're going to be their tech support for life. Ugh.
Or your client can get a
Dell Dimension 4400 for a little more with 24/7 tech support.
Processor: Pentium® 4 Processor at 1.6 GHz
Memory: 256MB DDR SDRAM
Keyboard: Dell® Enhanced Quiet Key Keyboard
Monitors: Video Ready w/o Monitor
Video Cards: 64MB NVIDIA GeForce2 MX 4X AGP Graphics Card with TV-Out
Hard Drive: 40GB Ultra ATA/100 Hard Drive
Floppy Drive: 3.5 in Floppy Drive
Operating System: Microsoft® Windows® XP Home Edition
Mouse: MS IntelliMouse®
Network Card: No Network Card Requested
Modem: 56K PCI Data Fax Modem for Windows
CD or DVD Drive:
FREE Upgrade! 24x/10x/40x CD-RW Drive with Roxio's Easy CD Creator®
Sound Card: Integrated Audio
Speakers: New Harman Kardon® HK-206 Speakers
Software Bundles: Microsoft® Works Suite 2002 with Money 2002 Standard
Virus Protection: Norton Antivirus® 2002, 90-day introductory offer
Digital Music: Dell Jukebox powered by MusicMatch
Digital Photography: Dell Picture Studio Image Expert® 2000
Limited Warranty, Services and Support Options: 1Yr Ltd. Warranty- 1Yr At-Home Service + 1Yr Phone Support
Internet Access Services: 6 Months of America Online Membership Included
Mail-In Rebate Offer: Save $125 with mail-in rebate. Price shown before rebate.
$819
-$75 off $800 coupon (expires TODAY!)
-3% eBates.com
-$125 MIR
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$596.68 after the dust settles