you said you want to accelerate your conversion using ur gpu according to ur choice nvidia is best and ou should go for it coz it has to use cuda cores for conversion but as coming to ur i7 upgrade i think there are enough processors for i7 to handle very fast conversion i always convert videos to different format and i always get the best quality in very quick time coz of my i7 860 getting nvidia is akso best as we come to conversion quality use divx converter best of all i found until now it may take time but doesnt compromises quality good luck
i'm no expert in this encoding stuff but isn't that intel quick sync (which was first introduced with sandy bridge processor) are much faster in this task? also all geforce 8000 series and above have "CUDA" cores inside them. if you look hard enough you can get GTX460 which a lot better than 550Ti for the same or less cost
The problem with video conversion/encoding quality is not the hardware used, but the software!
You need to get better software! You already have decent hardware, an i5 already speeds up encoding, though you will get better encoding performance with a decent nvidia or amd gfx card and an i7 cpu, you will not necessarily get better quality, that depends on your software used and the codec format; H.264, mpeg etc...
Hope this helps you decide.. remember that we are daily improving on technology and the speeds at which things are done! You can build a super mean system for encoding etc, but it will only be as fast as the software giving instructions and making of use of hardware provided... at the end it all depends on the software... if you take a look at gaming, some games run with excellent visuals with low hardware requirements, where others run also with excellent visuals but eats at low end hardware and systems. Its al the software..