1) Follow Anti-Static Procedures
Your body carries Static electricity. Static WILL fry out (Short Circuit), the delicate hardware components inside a computer.
Relieve your body of Static, BEFORE reaching inside your computer.
Computer on a table, computer Unplugged from power, computer case open;
TOUCH an unpainted surface, of the metal frame of the open computer case.
This action will relieve your body of Static.
IF, you leave your computer in the middle of working on it, be SURE to Touch the metal frame again; upon your return.
(Also use Anti-Static Procedures BEFORE removing new hardware, out of their antistatic bags, or cartons. Such as a graphics card, for example )
Now we come to;
1) Are you still using the graphics the computer came with?
According to this website it is an ATI Radeon X1300 graphics card,http://www.cnet.com.au/dell-dimension-e521_specs-339271259.htm
(The Integrated Graphics on the motherboard, is Nvidia GeForce 6150LE,http://support.dell.com/support/systemsinfo/document.aspx?s=dfh&~file=/systems/dime521/en/sm_en/specs.htmhttp://en.wikipedia.org/wiki/GeForce_6_Series#GeForce_6150LE
The graphics expansion slot you have is a PCI-Express x16 slot.
It is based on the Nvidia nForce 430, and Nvidia GeForce 6150LE; motherboard chipset,http://en.wikipedia.org/wiki/Comparison_of_Nvidia_graphics_processing_units#GeForce_6_.286xxx.29_series
Came out on the market June of 2006.
The PCI Express 2.0 technology didn't come out, until
January 15, 2007,http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_2.0
Therefore, we can assume the PCI Express technology your computer is using, is PCI Express 1.1
You can use a PCI Express 2.0 graphics card, though.
This is is because PCI Express 2.0 technology, is Backward Compatible with PCI Express 1.1 technology.
"PCIe 2.0 motherboard slots are fully backward compatible with PCIe v1.x
PCIe 2.0 cards are also generally backward compatible with PCIe
1.x motherboards, using the available bandwidth of PCI Express 1.1.
Overall, graphic cards or motherboards designed for v2.0 will work with
the other being v1.1 or v1.0a."
(From the above Wikipedia article)
However, will you get the full capability of the PCI Express 2.0 graphics card?
Will you notice?
You can also use a PCI Express graphics card, based on the PCI Express 2.1 technology.
1) PCI-Express x16 slot.
You have that.
(The Lock lever on the end, looks as though you press down, to unlock.
Actually you LIFT UP, slightly.
Use CARE though, and do NOT lift too much. You'll see it just takes a little. It's also PLASTIC, and can break.
Lift up slightly to install a graphics card, on remove one )
2) Power Supply:
You need to look at the system requirements. Plus more specifically, the Power Requirements.
Website that sells the graphics card does Not state this?
Go to a different website that does.
Your Power Supply is rated at 305 Watts.
HOWEVER, this power supply was made back in the day, when Power Supply manufacturers were 'fudging'; the REAL actual maximum rated Wattage.
Actual maximum rated Wattage is more like 60 to 70 percent, of what is stated.
305 Watts divided by 10 = 30.5
Round it off to 30.
30 times 6 = 180 Watts
30 times 7 = 210 Watts
It really depends on what you wish to do, with the graphics card.
It also depends on if you are willing to get a better Power Supply.
It also depends on the Processor.
AMD Athlon 64 X2 3600+?
Dual core processor that can operate Up To, a maximum frequency rate ('Speed'), of 2.0GHz.
(Can also be stated as 2000MegaHertz.
Going to need a 400 Watt power supply, though.http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=2941922&CatId=3669
500 Watt power supply.
Gives you more, than the recommended 10 percent extra power you should have.
This way the Power Supply will last longer, because it doesn't have to work as hard.
For additional questions please post in a Comment.