well u might wanna upgrade ur mobo to a 775 (P5B) then buy a Intel core duo on it and get 2 gigs of ram and maybe a Ati fire gl. that should get u started for.
ATI FireGL V3300 (PCIe) 199$ from newegg
Intel Core 2 Duo E6300 Conroe 1.86GHz 2M sharing L2 Cache LGA 775 Processor 183$
ASUS P5B Socket T (LGA 775) 139$
pqi POWER Series 1GB (2 x 512MB) 240-Pin DDR2 SDRAM DDR2 533
Ok ok i now this goes waaaaaaay over the budget but u could sell ur old pieces and buy this config...
NAd since u said video rendering i thot ud go with a fire gl or a matrox or quadro... might be wrong.. but thats just to get u started...