He is using a "full HD" monitor. I would assume that means at least 1080p. I don't think the HD5750 is an appropriate recommendation. Even with his current processor there are tons of games that will benefit from a better card at high resolutions. HD5770 should be the minimum IMO and a GTX 460/HD6850 optimal.
A 5750 is fine at 1080p, especially for somebody on a budget and who has been used to a 9600GSO - that's like what a 4x upgrade?
A 460 or 6850 will skin him and he'll be further from a better cpu + more RAM. However, I would go with my first recommendation, the 5770, so long as the price is right.