It's definitely not PCIe, because those Dell 4600s were before PCIe was added to common Dells. Same thing with DDR2.
I suggested doing a BIOS update, because this might allow you have 2GB of RAM. If there's no support, the machine might not boot, or use only 1GB. But the manual says the 4 slots support 1GB each, with dual-channelness. DDR-333 (PC2700) is what the memory speed is.
If you get matching pairs, it'd speed up the system a lot. 2x1GB would be ideal, but you could pop in another 512MB stick.
http://support.dell.com/support/edocs/systems/dim4600/So here's my buying suggestions:
A Radeon X1950GT http://www.newegg.com/Product/Product.aspx?ATT=14102034&CMP=AFC-SlickDeals&Item=N82E16814102034 There's a $20 coupon code (EMC907VGA10) but I can't get the discount to work.
3x 512MB DDR-333 (PC2700) http://www.newegg.com/Product/Product.asp?item=N82E16820141167 This'll bump you to 2GB total of relatively slow RAM.
Earthwatts 430W power supply 80%+ efficient.
http://www.newegg.com/Product/Product.asp?item=N82E16817371006 This seems like enough power.
$284.95 before shipping + taxes (if you're unlucky). There's a $20 rebate on the power supply.