Re: New cards in old AGP slots
I wouldn't recommend putting a new card on this motherboard. Your fx550 is equivalent to the geforce4mx. There are quite some other AGP cards like the geforcefx series and even members of the geforce6 and geforce7 series are available in AGP. The problem is that to take advantage of these faster cards you need a relatively fast cpu. Second your motherboard still uses old SDR memory which also holds performance back a lot.
I would recommend you to upgrade motherboard, cpu and ram. If you do that I expect that the videocard you choose won't matter much for your purpose. Even the onboard geforce6 class GPU on Geforce 6100/6150 motherboards can fullfill your needs.