For CS5 CUDA support,
the Mercury Playback Engine requires the NVIDIA video card to have at least 896 megs memory or more.. Adobe's list of officially supported cards is very limited, but a very simple .ini file edit allows you to add any recent nVidia card, provided it meets the above requirement. Hence a GTX460 768MB won't work for this (provided you actually use CS5 at all
). If you're not using CS5, then this is a great card. Highly recommended.
For speed difference:
1GB model:
http://www.anandtech.com/bench/Product/177?vs=180
768MB model:
http://www.anandtech.com/bench/Product/177?vs=156
It's distinctively faster, up to x2 the speed. This is provided you have a decent CPU and can find one that meets your price criteria. If you can't find one, go for a GTX550Ti I'd say. A bit slower and cheaper than a GTX460. If that's too much, there's also a GTS 450, has 1GB of RAM and is still faster than your HD4850. Plenty of models for ~$100. For a similar performing Radeon card, look at HD5750/HD6750 or the slightly faster HD5770/HD6770. They had their name change recently and are part of the 6-series, but you can find the "old" 5-series models (which are pretty much identical) on sale, like this $95 AR HD5770:
http://www.newegg.com/Product/Produc...-873-_-Product
Speed difference:
HD5750:
http://www.anandtech.com/bench/Product/177?vs=173
HD5770:
http://www.anandtech.com/bench/Product/177?vs=172