I read where the upgrade point was $300 and that may be close. You can build a basic AMD x2 computer for around $500. Here is a sample computer build I made at Newegg.com using their wish list feature.
DYNAPOWER USA Titan C05.N17.M158 Black SGCC Steel ATX Mini Tower Computer Case 430W Power Supply - Retail
Model #: C05.N17.M158
Item #: N82E16811138074
$39.99 $39.99
ASUS M2N-MX SE Plus AM2+/AM2 NVIDIA GeForce 6100 Micro ATX AMD Motherboard - Retail
Model #: M2N-MX SE Plus
Item #: N82E16813131249
$54.99 $54.99
EVGA 256-P2-N615-TX GeForce 7600GT 256MB 128-bit GDDR3 PCI Express x16 SLI Supported Video Card - Retail
Model #: 256-P2-N615-TX
Item #: N82E16814130062
Mail-in Rebate
$89.99 -$5.00 Instant $84.99
AMD Athlon 64 X2 4600+ Windsor 2.4GHz Socket AM2 89W Dual-Core Processor Model ADA4600IAA5CU - OEM
Model #: ADA4600IAA5CU
Item #: N82E16819103750
$69.99 $69.99
Update Kingston HyperX 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory Model KHX8500D2K2/2G - Retail
Model #: KHX8500D2K2/2G
Item #: N82E16820134218
$159.99 -$80.00 Instant $79.99
Western Digital Caviar RE WD2500YS 250GB 7200 RPM SATA-300 Hard Drive - OEM
Model #: WD2500YS
Item #: N82E16822136055
$69.99 $69.99
SAMSUNG 20X DVD±R DVD Burner Black SATA Model SH-S203B - OEM
Model #: SH-S203B
Item #: N82E16827151153
$29.99 $29.99
Windows XP Operating system OEM
$89
Subtotal: $518.9
This would be a very basic computer. However it outperforms your AGP computer in every area.
