After a while with the problem with my 350w PSU, I narrowed down to two video cards I want for Dell XPS 8100
Here are the specs for my PC:
Intel® CoreTM i5 650 (3.2GHz/4MB cache)
Windows® 7 Home Premium 64-Bit
Up to 3GB Dual Channel DDR3 SDRAM at 1333Mhz - 3 DIMMS
500GB SATA hard drive (7200RPM)
NVIDIA G310 GeForce®
350W Power Supply
I have doubts about the Nvidia 450 GTS because of its higher power supply requirements even though some people says the PSU on my PC is underrated.
The 5670 is the safer option, and not a bad card at all. It doesn't require an external power connector, and will run fine on a 350W PSU. I'm not too sure of the power requirements for a 450, but mine runs fine on a good 400W PSU so if your Dell PSU is indeed under-rated then it definitely has the possibility to be fine as well. Malmental is the man to ask about Dell machines!
One of the options when purchasing an 8100 is an upgrade to an ATI 5770, not that you would want to pay Dell's premium for upgrades.
Look at the power consumption chart above and decide if a 450 would work.