Graphics Card Upgrade

After some reserch on the dell website I found out that you pc is a small form factor, but it does have 1 PCI-E X16 connector, the only thing that you would be limited by would be the form factor and the power supply.
Edit:
just to make sure this is the computer:http://www.dell.com/us/p/inspiron-580s/pd ?

First question, Do you have a preference on whether you want Nvidia and AMD(ATI)
Will it be used for gaming? Movies? Home theater PC?
If for gaming do you want to play new games or games that are less intensive and are a 2-3 years back?

Okay, There is 1 problems: first the power supply is only 250 Watts which is a problem, If you would be willing to chang the power supply you could fit a Nvidia GTS 450 which is a very decent card with a decent price:
Newegg.com - Palit NE5S4500F0601 GeForce GTS 450 (Fermi) 1GB 128-bit GDDR5 PCI Express 2.0 x16 HDCP Ready Low Profile Ready Video Card

So the major problem is your power supply. If you want to run newer games, then you need a new PSU, but if you do not want to upgrade the PSU you will most likely be stuck with something like a: HD 5450 ATI card.

hi there :)
the ATI 5000 series are good i use the XFX HD5550 fantastic budget card no external power needed it is powered from the board i run two lcd/led monitors with no problems with games/movies/tv etc. ionbasa is correct you could increase the psu to 500w.

Radeon HD 5550 Key Features

– ATI Eyefinity technology with support for
up to three displays (1,4)
– ATI Stream technology (2)
- Designed for DirectCompute 11
and OpenCL(6)
- Accelerated Video Transcoding (AVT) (2,5)
– Compliant with DirectX® 11 and
earlier revisions
– Supports OpenGL 3.2 (6)
– ATI Avivo™ HD video and
display technology (3,7)
– ATI PowerPlay™ technology (7)
– DL-DVI, DisplayPort, HDMI
– PCI Express® 2.1 support
- Low profile graphics card, perfect for small form factor or home theater PCs

LINK Radeon HD 5550