The following config should be fast enough for your tasks and low cost and low power consumption.
Sempron 140 - 1.7K approx (single core but fast, cheap, low TDP)
ASUS M4A78LT-M LE - 2.9K approx iirc
(Socket AM3, HyperTransport 3.0, 760G, DVI port, EPU for power saving, 6x SATA ports, Anti-Surge protection, Express gate, ...)
ASUSTeK Computer Inc.
2GB DDR3 1333MHz RAM, Corsair- 2K approx
Seagate 1TB HDD- 2.8K approx
FSP SAGAII 350 - 1.5K approx
CM Elite 310 case - 1.6K approx
keyboard+mouse - 0.5K approx
APC 600VA UPS - 1.8K approx
20" LCD- 6.5K approx
Total- approx 21.4K (approx 14.9K without LCD)
You may reduce the cost a bit by getting a 17" or 18.5"/19" LCD- if you need a LCD monitor within 20K too.
To get a rough idea on prices- *www.deltapage.com/list/index.html