trying to verify my power supply calculations, graphics card advice

I have a Dell Studio Slim Desktop computer, with an enemic, and as yet no upgrde available PSU>

Studio Slim-Tower, Intel Core 2 Quad Processor Q6600 (2.40GHz, 1066MHz FSB), w/ 8MBcache
6GB DDR2 SDRAM at 800MHz - 2X2GB+2X1GB
500GB Serial ATA II Hard Drive(7200RPM)
Integrated 10/1000 Ethernet
No Modem Option
16X DVD+/-RW DRIVE,BLK,DIM,M
Integrated 7.1 Channel Audio
Basic Dell fan.
250 Watt DC Power Supply
usb mouse, keyboard.

Dell says I can only use a VisionTEK Radeon X1300 DMS59 256 MB GDDR2 SDRAM PCI-E 16 Graphics Card due to the 250 w power supply.

I'm looking at the following vide card (has to be low profile card)

XFX GeForce 8400GS 256MB DDR2 DMS-59 Dual Monitor Low Profile Solution PCI Express 16 Video Card

according to various power supply calculator websites,like http://extreme.outervision.com/psucalculatorlite.jsp
they indicate I can use the above card, only reaching 238 watts.

I also selected hgh end motherboard, and the largets fan size as I wasnt sure about those, so i went on the side of cuation.

New EGG says I can't--but then the new egg calculator says I need 400 watts for my system without any video card.

The 1300 is cheap, but its a hunk of junk from what I read in reviews. The XFX GeForce 8400GS seems to be far better, and cheaper, but it claims to need a 300 watt PSU


I'm not expecting cutting edge, I just want decent performance watching a movies, online videos and the ability to play some games on average video settings, not high end settings. (my main games are City of Heroes, Dawn of War, would like to try Spore).

What would you most excellent and knowledgeable individuals suggest? This is my first attempt ever to upgrade a video card, so please pardon my stupid questions or any vital information I have failed to list. It's by lurking the forum I learned about power supply calculators, why I needed a low profile card, and many other useful bits of information, so I'm already indebted to the forum. Like a good consumer, I'm now going to add to that debt. Thank you very much in advance.

Thank you for the link to that thread. Awesome information.