PSU/ Video card question

I bought a dell Inspiron 537, 3.0ghz Core 2 duo & 2 gigs of ram... I want to put a Evga geforce 8800 gts KO edition in it & a better PSU cus the one in it is only 250watts...

I'll be running the E8400, 320 gig hard drive, 2 gigs of ram & that 8800 GTS

My question is, what size PSU would I need, i calculated it and it came to be 399 watts.. so should I get a 450-500 to be safe? And how hard will it be replacing the PSU in a DELL computer? I've built a couple myself but never actually replaced a PSU, will I have to take the MOBO out? I think it should be a simple swap but im not sure

Also, this 8800 got good reviews and said it'd play most anything out on HIGH settings, true?

Im wanting to do this and give it to my brother for his birthday, Monday, since he's got a 5 year old dell and cant play the games he wants anymore.


Thanks guys.

Thanks!

Will that card play modern games on good settings?

and someone said they were playing Witch, COD5 on High and crysis on medium with that 8800..

Would the 9800GTX+ be better than that ATI card?

All I know about the dell is it's a Mini Case,. I cant find what size PSU is in it and when I call them, none of them speak english!!

It's a Mini Tower case.

here are the deminsions:

Dimensions
Height: 37.53cm (14.8" )
Width: 17.59cm (6.93" )
Depth: 44.22cm (17.44" )

I dont have the computer yet, heres the only inside pic I could find.

Thanks a lot!

and I know it'd been better to build but we are tight on cash now, with half the people here being laid off so we went with dell being we have an account there and get make payments.

but agian, thanks for the help!!