Sorry but I cannot give any advice on the difference between 4350 and 4650, However yesterday I installed my sapphire branded 4670 and although its not loud you can hear the fan noise at idle, not irritatingly loud, but is is there in the background.
Having had a quick look on ebuyer all the 4650's seem to have fans so this is something you may wish to consider, whereas a number of the 4350s are passively cooled.
Obviously a 4650 would be more powerful, but if your HTPC is completely silent and space is tight, this may be a purchasing decision.
A 4350 is more than ample for HD playback, and you'll not notice any benefit in the 4650 unless you were gaming.
The 4650 is available passively cooled, but not as widely available as the 4350 (as noted above) - and the 4350 will also run cooler and draw less power.
In short, if you're not gaming whatsoever, save the £15 and get a takeaway or a nice bottle of wine or two to enjoy a movie with.
Regarding a 4650 versus onboard IGP. You'll probably notice a difference with interlaced content, unless you're using a Phenom CPU OR you have a motherboard with dedicated 128MB video memory onboard). For example, if you are watching SD TV broadcasts from DVB-T or DVB-S, or TV show DVDs you might notice an improvement. This is because the memory bandwidth available to the IGP when using a HT 2.0 CPU (i.e. Athlon X2) is insufficient for vector adaptive deinterlacing at high resolutions and may even fall back to Bob deinterlacing at high resolutions (it does this on my 690G IGP).
But for Blu-ray playback or movies on DVD, you shouldn't see much, if any, difference.