You do not need to fill all the slots. You do need to follow the user manual as to which slots to plug the ram into tho. Usually the slots are color-coded. For example, if a board has 4 slots, 2 will be one color, 2 will be a darker color - they are also numbered, and usually staggered. 

investing in a high quality board that will last you a while and is capable of expansion would be the best way to go, IMO, and save you money in the long run.

This is the board I have: Asus Sabertooth 990FX AMD3+

http://www.newegg.com/Product/Product.aspx?Item=N82E16813131736

It's a very well made board. It may be a bit out of your price range at the moment (I don't know what your budget is) but for me it was well worth the price. I got it on sale for a bit cheaper than that about 6 months ago, and I shouldn't need to buy another board any time in the near future unless something unseen happens to this one. 

Also, the board size is extremely important based on what case you're putting it in. Of course a case can be modified to accept various boards, but it's not recommended unless you really know what you're doing, as it is not difficult at all to short out a mother board if its installed incorrectly as the screw holes are for mounting as well as grounding.

For graphics and rendering, you want lots of air flow to keep the components as cool as possible. The more ram and processors you have the more heat your machine will generate, so you want to make sure your case is large enough and you have several fans running. There are cases that are designed specifically for high air flow, and cases really aren't that expensive. But the more fans you have the higher your noise levels will be. Its necessary for high powered machines, so it's really a trade-off. Which is more important based on your needs.

 

~Shane