Nicolas, the Radeon X1650 Pro for your Dimension 5100 does indeed gets all of it's power from the PCI-Express slot. As for your Dimension 4600, make sure that the 4-pin Molex power connector cable being used for the Radeon 9800 Pro has no other device plugged into the same cable.

Does your Dimension 4600 have a PCI card installed in the uppermost PCI slot like the one seen in the photo you posted? If so, that could be hindering effective cooling of the Radeon. In my machine, my Sound Blaster X-Fi XtremeMusic PCI card is installed in the bottommost slot (PCI3) to give my video card plenty of room for ventilation. There is also a 120mm fan mounted on the hard drive cage in my Antec Sonata case that keeps air constantly moving over both cards.

I'm under the impression that PSUs with a bottom-mounted 120-140mm fan draws air from inside the computer case and exhausts it out the back of the PSU. My Antec TruePower Trio 650 does this. For my next PC build, using the Antec P180B case which has a bottom PSU chamber, I selected the PC Power & Cooling Turbo-Cool 510 SLI PSU, which has a rear-mounted 80mm fan.

What is the +12V rail rating on the 420W Trust PSU? I can't locate that piece in info. The 450W Cooler Master PSU I mentioned in your other thread has a continuous +12V output of 22A (combining both +12V rails). And PC Power & Cooling's 410W Dell solution has a single +12V rail rated at 23A. Anyway, the Trust unit should be OK for the 9800 Pro.