Hi,
Welcome to the forums.
Well if you are going to be doing a lot of rendering, RAM is your main contender that you need to concentrate on, and ofcourse a good graphics card too. A 256mb ATI card should be fine.
I presume your new motherboard will be PCI-E ready so look at PCI-E graphic cards.
For the RAM......If I was you I would sell the 512mb you have on ebay and get yourself a stick of 1GB.
Do this becuase 1) it will mean less slots are taken up by loads of 512mb ones, and 2) if you want to stick in another 1gb too, it will run perfectly nicely in dual channel with the other one in there.
With that amount of money you should have no problem getting what you need.
As for sites I'd reccomend..... For memory:
http://www.crucial.com/uk/
Just choose your motherboard and it will find you the right type of RAM.
And for PC parts, I tend to use
www.overclockers.co.uk