If it has set for 3 years the CD/DVD drive is shot or will die shortly after you boot. The rubber parts will have dry rotted and any lubricated parts will have become sticky. The HDD may fare better, because it is sealed, but it should be replaced also as it is....old and IDE. I wouldn't rely on it. which leaves you with the CASE and PSU. the PSU should be replaced for the above mentioned reasons also. the case itself is reusable, but i would replace the fans, they are fairly inexpensive. That being said I will present you with 2 build options. the first is the best, but require you to upgrade your OS to WIN7 (assuming you currently run XP) the second allows you to use your current OS (XP) with a future upgrade to WIN 7.
Core i3 Sandy Bridge build. (uses Intel HD Graphics 2000) allows future discrete video card upgrade:
LITE-ON CD/DVD Burner - Bulk Black SATA Model iHAS124-04 - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16827106289
$19.99
SAMSUNG Spinpoint F3 HD502HJ 500GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16822152181
$49.99
Antec EarthWatts EA-500D Green 500W ATX12V v2.3 / EPS12V 80 PLUS BRONZE Certified Active PFC
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16817371035
-$30.00 Instant
$89.99
$59.99
Mushkin Enhanced Essentials 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel Kit
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16820146748
Return Policy: Memory Standard Return Policy
$39.99
Microsoft Windows 7 Home Premium 64-bit 1-Pack for System Builders - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16832116754
Return Policy: Software Standard Return Policy
-$5.00 Instant
$99.99
$94.99
http://www.newegg.com/Product/ComboDealDetails.aspx?Ite...
Intel Core i3-2100 Sandy Bridge 3.1GHz LGA 1155 65W Dual-Core Desktop Processor BX80623I32100
Item #: N82E16819115078
MSI H67MS-E43 (B3) LGA 1155 Intel H67 HDMI SATA 6Gb/s Micro ATX Intel Motherboard
Item #: N82E16813130584
-$20.00 Combo
$234.98
$214.98
--------------
$479.93
AMD build allows you to use WIN XP (hopefully you floppy drive still works) uses ATI HD 4250 Gaphics
LITE-ON CD/DVD Burner - Bulk Black SATA Model iHAS124-04 - OEM
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16827106289
$19.99
SAMSUNG Spinpoint F3 HD502HJ 500GB 7200 RPM SATA 3.0Gb/s 3.5" Internal Hard Drive -Bare Drive
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16822152181
$49.99
Antec EarthWatts EA-500D Green 500W ATX12V v2.3 / EPS12V 80 PLUS BRONZE Active PFC Power Supply
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16817371035
-$30.00 Instant
$89.99
$59.99
Mushkin Enhanced Essentials 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Dual Channel
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Item #: N82E16820146748
$39.99
MB/CPU Combo
http://www.newegg.com/Product/ComboDealDetails.aspx?Ite...
AMD Phenom II X4 840 3.2GHz Socket AM3 95W Quad-Core Desktop Processor HDX840WFGMBOX
Item #: N82E16819103921
GIGABYTE GA-880GA-UD3H AM3 AMD 880G SATA 6Gb/s USB 3.0 HDMI ATX AMD Motherboard
Item #: N82E16813128444
-$25.00 Combo
$219.98
$194.
---------
$364.94