the 6750 is the more powerful of the two you listed.
what is your budget?
what are the specifications of the pc you're gonna use the gfx card?
which site will you use to buy/choose gfx cards?
how many amperes do your psu's +12v rail have? or psu model. <- important.
a good 450 w psu should be able to support up to radeon hd 6770 and 6850.
I'd usually say try and move up to a 6870 (122 watts) .... $140ish but AMD lists it as requiring a 500 watter (same for 6790), . ..... a quality 450 should suffice. The 6770 AMD lists as requiring a 450W
my power supply is zebronics 450w and it is marked 18 under +12v,i decided to buy from flipkart.com,my pc specs are core2duo e7400 2.8ghz,4gb ddr2 ram 666mhz,windows 7 ultimate ,motheboard is intel dg31pr, pci-e x16 2.0 slot is provided
zebronics....
upgrade your psu to a quality one from antec/seasonic/enermax. make sure your psu has 80+ rating and lots of amps on its +12v rail. you can support higher end cards with that.
max, radeon hd 6670 with 1 gb gddr5 vram.
with a better psu, up to 6770 or 6850.
edit: the psu should be continuously able to supply power to the gfx card. i suggest a psu upgrade first, then gfx card.