i have built a number of machines over the years and find most suppliers effecient. there are of course exceptions to every rule and the same goes for this.
i would recommend you read the forum to see other peoples experience with on line retailers and then make an informed decision on who you buy from. iwould also say that you should shop around as different companies have different specials on to grab your attention. if you are not sure about a supplier do a search on this forum for any old postings regarding people gripes and you will see a pattern emerge.
if you want a personal recommendation and i stress it is personal then i would say click here or click here
aslo go to crucial for your memory as they have a handy tool on their site to help make sure the ram will be compatible with your motherboard and they garuantee it will work.
sood luck and watch out that you get everything that compatible and if you are not sure ring the company and ask
ps. use a credit card!!