This is purely from my own experience, 'k?
Bestest online supplier evah: Kustom PC's.
I also use, in order of preference:
Scan
E-Buyer
Aria
Chillblast
I've had one problem with Scan, a little too detailed to explain but basically they once accused me of rma-ing a component where the serial number, according to them, didn't match and they wouldn't refund or replace. It was touch or go for a while but in the end they confessed their mistake and I had £150.00 reinstated to me. That was scary as hell, really.
Despite that I get free postage with Scan as I'm a member of Hexus forums with over 50 posts and their delivery process is really excellent. Prices are quite fair too.
They don't specialise much but if they stock what you're after, then it's good.
Just watch out for them trying to charge extra for some supposed extra warranty, you have to watch out to untick the box, that, to my mind, is just blatant cash extortion on Scan's part.
Aria have been good but don't believe their special offers, they sell out in about ten seconds, lol, a blatant con. Aria rma-ed me a faulty NIC card within 2 days turnaround once, which was good.
E-Buyer have good prices and refunded me for memory that had gone faulty with only one week of warranty left once, I'd trust them.
I haven't used Chillblast for a few years but way back in the day they were good, easy communication.
Both Chillblast and Kustom PC's charge a little more than all the others I've mentioned here but with Kustom PC's it is most definitely worth it.
But remember - this is just MY experience.