Upgrading graphics card, need advice

I didn't know they made a low profile HD3850. You'll have to upgrade your power supply for it. This Seasonic SFX 350w should work, it has the amps for it. Make sure it's the right size to fit in your case before you buy it.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Sorry the graphics card that I'm wanting is the HD 4850 not the 3850. Also what would happened if I used the the 250w power supply, even though the required power for the card is 450w.

Does your PSU have a 6pin PCI-Express connector? What resolution is your monitor? What CPU do you have?

Yes my psu is a 6 pin pci-e, my max resolution is 1680 x 1050, and my cpu is a dual 2.53 ghz. My computer meets all the requirments on the box except for the power supply, which is 450w while my computer's power supply is 250w.

How many amps on 12V?

I'm sorry I'm a bit new to this what do you mean.

The slim case from Dell uses a TFX form factor power supply. There aren't many you can find, but I'd suggest getting this one http://www.newegg.com/Product/Product.aspx?Item=N82E168... from Seasonic (they make great quality power supplies). It's the highest wattage one Newegg carries, although even then you're only looking at 50 watts more than what you have already......and you'll need to buy a molex to PCIe 6 pin adaptor (such as http://www.newegg.com/Product/Product.aspx?Item=N82E168... or http://www.newegg.com/Product/Product.aspx?Item=N82E168... )

Also, you'll need to to buy a what's known as a "low profile" graphics card. As the name suggests, it's smaller than the normal size cards. As far as I know, there aren't any low profile HD 4850's out there. Do you have a link to the card you want to install?

The top performing low profile card newegg carries would be this 9800GT http://www.newegg.com/Product/Product.aspx?Item=N82E168... It's no 4850, but it isn't too far off. The runner up would be the 9600GT, http://www.newegg.com/Product/Product.aspx?Item=N82E168... or http://www.newegg.com/Product/Product.aspx?Item=N82E168... . The 9600GT's would probably work without having to replace your current power supply, as the 9600GT isn't usually too power hungry.

The 9800GT is a good choice if you choose to go for it. I'd get it over the 9600GT. I've seen the 9800GT work with 250W power supplies ( http://www.pchangout.com/forum/video-cards/39578-geforc... ) so I'm pretty confident that it will work with the 300W seasonic I mentioned. It may even work with your 250W, but I wouldn't recommend it. Your mileage may vary though