250 is too low.
I would suggest at least upgrading to a 300W minimum or better power supply.
How many Amps is there on the +12V rail?
If you are tight on budget I would recommend going with HD 6570 or 6670.
AMD/Ati cards such as HD 6670 is pci-express 2.1x16
I heard that 2.1 cards don't work with old motherboard which is 1.0(the motherboard you currently have)
GT 440 is pci express 2.0x16 card which currently does not have any issue with old pci express 1.0x16 motherboard.
Make sure you get the GDDR5 version- http://www.newegg.com/Product/Product.aspx?Item=N82E168...
If you want smooth gameplay you may need to lower some settings or play at 1280x720.
Upgrade the power supply first just to be safe.
Yes, PCI-E 2.0 cards will work on PCI-E 1.0 motherboard.
The only issue i see is PCI-E 2.1 cards on PCI-E 1.0 motherboard.
I heard issues that people encounter when using PCI-E 2.1 cards on PCI-E 1.0 motherboard.
The GT 440 will work fine since it's a PCI-E 2.0 card.
14A*12V= 168W for the +12V rail.
This does not include the motherboard and hard drives.
If you plan to buy a PCI-E 2.1 cards it should be 50/50 chance that it will work with your motherboard.
PCI-E 2.1 card- http://www.flipkart.com/xfx-amd-ati-radeon-hd-6670-1-gb...
Acording to my knowledge the pci x16 1.0 is same as 1.0a is same as 2.0 is same as 3.0. According to me only chipset drivers are configured according to the capability of the card installed on this port. Senior members !! Clarification needed.