Replacing GPU on a 27" 2010 imac i3 3.2ghz (A1312)

I Have a 2010 27" IMac i3 3.2ghz (A1312) ver 10.6.7 which the ATI Radeon HD 5670 512 MB of GDDR3 died (confirmed by apple tech). I can replace the 5670 with a refurbished unit for $300 or up grade to the ATI Radeon 5750 1 GB GDDR5 SDRAM ($450 New). All of what I have read on the web and from Apple the 5750 should be an easy swap. This apparently is one of the few time you could have upgraded a GPU with my above configuration. Being a skeptic at heart I wanted a real world opinion from all of you. Have any of you upgraded or replaced the GPU in my configuration. And second is it that big of a difference between the HD 5670 and the HD 5750? My 92 year old aunt bought it new and only drove it to church on Sunday. She bought a new 27 and gave it to me a week ago, then it died. It was a freebie and I thought if I could juice it up a bit, my kids would have ok game and school unit.

Thanks all,

Tom