Man, i've been searching and guess what! I had an old dimension E521 back in 2007 or 2008. It came with intel core 2 duo, 1gb ram, and one Radeon X1300 PRO. I tried to upgrade the VGA and bought one XFX 8600GT fatal1ty 1GB, but it didn't worked i don't know why. I sold the VGA and changed the PC for a dell XPS8300 in 2011. I would recommend to you to invest in a new PC, if possibly. If not, update the BIOS in the DELL website. I am not sure, but the intel core 2 duo might bottleneck you, and you should get and AMD Athlon x2 6000 wich should work if you update your BIOS. The recommended PSU wattage of the HD4670 is 400w in the amd website, and the standard PSU of the E521 is 305W, so i don't know, it might work (i didn't found the card max TDP) but i recommend a new PSU, too. The Radeon 4670 ICE QQ is dual slot, and the case cannot fit a dual slot card unless you take off the rear support plate on the card. You should buy this VGA: http://www.newegg.ca/Product/Product.aspx?Item=N82E16814161403.
If you need more help look at:
http://www.tomshardware.com/forum/320787-28-dell-dimension-e521-upgrade
http://www.tomshardware.com/forum/366640-28-dimension-e521-power-supply
http://www.tomshardware.com/forum/354475-33-dell-dimension-e521-video-card-upgrade
If i helped you, choose this message as best answer