Yeah you can have just one, and it will do the whole picture. I wouldn't go with XFX again, as I haven't been on my gaming PC in a month because of it overheating. Everyone here likes eVGA, because they have a trade in deal, and you pay the difference to get a new card. I have a BFG, and it is very nice. They are overclocked as standard, and have a lifetime warranty. Leadtek put HUGE all copper heatsinks on their cards (which is great
I personally wouldn't go with a Gigabyte motherboard, as they are just okay motherboards, nothing special. Not perticually great at overclocking. DFI lanparty boards are the best ones out there. MSI and ASUS are great companies too.
If you have some money left over 2gb's is starting to become the normal for gaming computers. It is best to get 2 1gb sticks over 4x 512.
I would get this over the heatsink you picked out: http://www.newegg.com/Product/Produc...82E16835118114
If you have more than there is a all copper one, which is better. Or if you really want: http://www.newegg.com/Product/Produc...82E16835118223
The CPU is great. The early ones overclocked like mad. 2.8ghz stable on air! Now you are unlikley to get one of them, as AMD originally had the 3000 exactly the same as a 3800, just a different multiplier.