Hi,
If your current power supply has 6 pin connectors on it your current power supply looks like it could meet the minimum wattage requirement. However, if you do need/want to upgrade, I would suggest a power supply that provides 500-600 watts which should give you room to upgrade your computer in the future. I think 2 of the 6770's running in crossfire mode they recommend a 600 watt power supply. You should also look for a power supply that has some kind of 80+ bronze, silver, gold, or platinum efficiency certification. These draw less power and save you energy costs as opposed to power supplies that do not have the ratings (meaning cheaper quality power supplies need to draw more power from the electrical outlet to produce the same level of power as one of these certified ones, meaning a lot more wasted electricity and money essentially). In my computers, I personally use thermaltake toughpower power supply and also pc power and cooling ones and have had no issues. Other brands that have positive feedback are Antec, SeaSonic, and XFX. Just be sure that the one you want has at least 2 or 2 6 pin pcie power connectors for your video card. As tempting as it might be, do not skimp on the power supply. The really cheap ones are usually cheap for a reason, most like because they use cheap parts which may or may not last, and sometimes when they go they can take out other parts of the pc, not just themselves, so its worth it to invest in a good quality one.
This one would work for you and is not too expensive and has many positive reviews on it:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016
I know you are on a budget so the above antec power supply combined with a 6770 would probably be as close as you could get to 150 dollars, but would still be over by maybe 20 or 30 dollars, but if you had the money, this is what I have in one of my computers now:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817153140
and this one in my other computer :
http://www.newegg.com/Product/Product.aspx?Item=N82E16817703027
Both have these have been running for about 4 years now with no issues.
You can do a search on newegg.com or tigerdirect.com for 500-600 watt power supplies to see what else is out there, I would recommended looking for one of the 80+ certification ones and be sure that it says it has 1 or 2 pcie 6 pin power connectors so you can hook up the video card :)
