Here's a couple builds I figured I would post. The first is showing you what you can get cpu-wise (quad-core) which would be ridiculously future proof for your needs. The second is a very cheap system that would do what you need to just fine as well. Maybe use one or both as a starting point for your build, concerning hard drives, case, etc.
1)Case Antec 300 $49.99 (microcenter.com)
Psu XClio GDPR 500W $54.99 (newegg.com)
Cpu Intel Q6600 $219.99 (newegg.com)
Mobo Gigabyte GA-G33M $94.99 (newegg.com)
Memory 2GB G.Skill CL5 $42.99 (newegg.com)
Hdd1 WD 160GB $44.99 (newegg.com)
Opt1 Samsung DVD-R $24.99 (newegg.com)
Total $542.93
2)Case Antec 300 $49.99 (microcenter.com)
Psu XClio GDPR 500W $54.99 (newegg.com)
Cpu Athlon X2 4400+ $60.99 (newegg.com)
Mobo Gigabyte GA-MA69VM $64.99 (newegg.com)
Memory 2GB G.Skill CL5 $42.99 (newegg.com)
Hdd1 WD 160GB $44.99 (newegg.com)
Opt1 Samsung DVD-R $24.99 (newegg.com)
Total $343.93
Btw, typing this post, surfing the internet, and listening to music currently...on a 733Mhz P3 just fine.