Check out this system for under $300.
http://www.directron.com/sysbackschool1.html
Processor: Intel Pentium Dual-Core E5200 2.50GHz 800MHz FSB, 2MB Cache.
Memory: 2GB DDR2 800 (2x1GB) dual channel memory.
Motherboard: ASUS P5QPL-AM MicroATX motherboard with Intel G41 Chipset.
Video: Onboard integrated graphics.
Audio: Intel High Definition 6-channel audio.
Storage: 160GB SATA Hard Drive
Optical: 22X SATA DVD-RW Drive
Case: Cooler Master Elite 330 RC-330-KKR1 mid tower case with 350W power supply
LAN: Gigabit (10/100/1000 Mbits/sec).
Edit: For a monitor, this week, Staples has a 21.5" e-machines 1920x1080 HD monitor for $100, and they also have a printable 10% off coupon, check their weekly ad for the week of Dec 20th. Between myself and a relative, we picked up three of those monitors.
So that leaves about $10 in the budget for some cheap speakers.