what is the brand of computer memory to buy?  

It depends on how fast you want your Computer to be and what will you be using this computer for, i have been building computers for about 7 years now and have used different types of memory RAM and there is a different thing about all of them, you should try to do a search for the right one then you will find the best, i think that if you visit the dell website then you will find a good bargain, try to get the Crucial sticks if you can but remember to keep the budget down because if for ever reason you want to sell this computer then you will not make half the money back that it cost to make the PC, also i would suggest that you try to use the same operating system that you have on your current pc and some of the parts so that you keep the cost down, good luck