Kind of what I expected from Dell. They're usually very good supporting their own components, but they seem to do everything they can to talk customers out of buying anything they can't sell to you directly themselves, even if it is compatible. I can tell you that the SL350 Antec 350W PSU definitely is compatible because I would not be typing this if it weren't.
Now I am about to prove my point above regarding Dell.
I have an account with Dell which has the Dimesion 4600 on it as the default system, as well as my g/f's PC which was purchased last year (both PC's purchased through the Dell.com website). When I login and select the 4600 and then select "Find More Parts for Your Dimension 4600", I get a dropdown box to select Dimension4600 with AGP graphics card or with integrated graphics. I select AGP (as would you, since we both currently have the NVidia FX5200 AGP card installed).
I am then sent to a new screen to select which category, such as power, memory, graphics cards, software etc. So I select graphics cards. Guess what appears on the list of compatible graphics cards? Here, I'll cut and paste it for you:
VisionTEK
XTASY Radeon X1300 512 MB DDR2 AGP Graphics Card
Usually Ships Within 24 Hours
Manufacturer Part# VTKX1300512AGP
Dell Part# A0601303
Interface Type: AGP 8x
Video Memory Installed: 512 MB
Starting Price $119.99
Interesting huh? That is the 3rd card on the list, and more than double what I paid for my one.
Believe me now?
Try calling back and say you want to buy a replacement graphics card *from them*, and say that you want the best one you can get that is compatible with the Dimension 4600, and see what they say then. Try not to choke when they quote the X1300, or when they quote their prices either.
FYI they didn't have any PSUs listed, just the 3V Lithium Coin System Battery. Maybe they meant the X1300 is not compatible with the stock 250W PSU.. but they sure have a funny way of saying it don't they....
Hope that helps.