This looks good to me, but you should still list it all out and post on the "new system build section" for input from others. They will inevitably get into arguments about $10 price differences on products, but I still think it would give both of us more peace of mind if others look at it. Be very clear with everyone about your intent -- a machine that doesn't require top level gaming performance, but that you will keep for a long time. You are looking for an adult machine with quality and reliability, not the best shooter FPS for the dollar like many on this site.
The 7900 does seem a little dated for everything else
. The Sapphire is a good choice -- they tend to be a good quality for the price.
You shouldn't need another heatsink for a non-overclocking machine unless you want something really quiet.
Make sure your optical drive has the right plug for the motherboard. I had to buy a new dvd burner when I upgraded because my old one wasn't SATA.
I'm currently using
G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL8D-8GBXM and they have worked great. (and are one of the highest rated items on newegg) This model has latency of CAS 8, but you can get cheaper ram (~$50) if you go up to the CAS 9 model as many people do. (In general, the lower the CAS latency, the better -- and many people cant tell the difference)
I'm not sure what to tell you on the motherboard, since it doesnt seem like you would need most of the features. It seems like one of the $130-$140 models would work fine for you. While 2 graphic cards seems unlikely in your future, there is a possibility of using a second PCIe x16 for some other type of add-on like a custom SSD card. (Maybe someone else will have more clear guidance here.) (I personally bought way more motherboard than I needed, but I do like having some security in knowing that I could install a lot more stuff in this one before it fills up)
You should also buy an SSD boot drive whenever you upgrade this machine next, maybe in a year or so -- their prices are falling fast (Crucial M4 is now at 128gb for $150) and they provide much faster boot and load times. (although your laptop may actually be the place where this would be most appreciated)