Dell XPS 400 w/ HD 4670 1G any possible upgrades?

Hey just as the title says I've got a Dell XPS 400 with a Radeon HD 4670 1GB 128-bit GDDR3(VisionTek if it matters) and was wondering on my power supply (which is the factory stock 375w ps) if I could run anything better. I've read in a few places that Dell underrates their power supply's. Anyways any help would be great. I bought this card well over a year ago and was just curious as to whether or not I could find anything better.

You PSU can run a HD 4850 easy...But have seen people pushing it by running HD 4870 too...But that is not much advisable...
http://en.community.dell.com/forum [...] 00564.aspx
And about the last post in the above thread, the HD 4890 is a revised 4870 but it draws more power than the HD 4870...

Do you think a Dell xps 400 with an upgraded PSU (to whatever), be able to use a eVGA GeForce GTX 260? Or what would you think could be run best, for gaming for such games such as Red Faction Guerrilla, Crisis, Fallout 3, etc... High end graphic games, and be able to run on The highest detail without too slow of framerate? I am looking to upgrade to at least a 550 or higher ps, and was wondering what is the best graphics card, that could fit and work just fine?

@ Anonymous: Either the HD4850 or GTS250 would be a safe choice, and a useful upgrade. For anything more powerful I would strongly advise getting a more powerful PSU: Antec, Pc Power and Cooling, OCZ, Seasonic, Corsair and FSP all come highly recommended.
@ RichDaMan: Start your own threads, hijacking others is d**n rude!