There would be a significant boost in performance, mainly because "6" is just a re-branding for next year (like "7 for "6" these days), thus "HD 6450" is basically "HD 5450" re-branded and "HD 5750" equals "HD 6750" (which is quite more powerful than your current video card).
Look at this
P.S. Too bad it's Diamond, though. :\
Because, even though HD 5750 is quite good by itself, "Diamond Multimedia" is ill-known for the crappy quality of the components they use in the building process.
Anyway, I'd still highly recommend getting your friend's HD 5750, it would give you a HUGE boost in performance, even if it means getting crappy "Diamond".
Get that HD 5750. Compared to your current video card, it's a totally worthwhile upgrade.