|Where do you live?|
Search for something suitable web site in your country. .
E.g. If it's in the UK
search for: graphics card uk
or - video card uk
Look for "hits" with uk, or whatever the code for your country is, at the end of the address.
seems to be pretty good - it shows you the minimum PS watts required in most cases
Find the listings for PCI-E Video cards or Graphics cards
Sort the list according to price - low to high.
Keep in mind when you order online, you usually have to pay for shipping on top of what you pay for a card.
You can usually buy the same or similar cards locally - no shipping charge if you pick it up - they often have web sites - look in your local phone book yellow pages or similar under Computer Parts or similar - the web address is often in the ad.
If you're willing to spend some more and upgrade your power supply, you can install ANY PCI-E X16 card.
If you're not, you need to choose a card with a chipset that requires the system have a power supply of 400 watts or less capacity. It's not recommended you get one that requires 400 watts, because that would be loading the PS to near it's full capacity.
E.g. Radeon 2600XT, NVidia 9500GT require a minium 350 watt PS - that would be a better choice for you.
Zotac GeForce 9500GT 512MB Cuda PCI-Express graphics card
Radeon 2600 XT - 300 watts minmum
9600GT, Radeon 2600 Pro, 2670, 3600 series, 4550, 4650, 4670 - 400 watts minimum
Radeon 3850, 3870, 4770, 4830, 4850 - 450 watts minimum
Radeon 4870 - 500 watts minimum