I've use novatech a lot, there online chat system is pretty good, And although i have never had to return anything I did have a suspect motherboard (turned out to be a bent connector pin when i checked) But they were more than helpful.
Aria have always been pretty good.
Watercooling UK are also good for bits, the delivery can seem a little slow at times, it normally takes 48 hours for them to pick the stock.
All the above companies are excellent with packaging.
The one i would avoid is eclipse computers. Even tho there pretty big, and i live in coventry (their location). I have had loads of problems with returns. They even lost a full hard disc and the paperwork. You have to fight them to get anything returned and that's going into the store. So i certainly wouldn't want to use them online.
Also If anyone has a pc check off list for building a pc
Start with CPU.
Motherboard (matched to cpu)
Ram (matched to motherboard and cpu)
Hard drive. (matched to motherboard)
Graphics card.
Case (matched to graphics card and motherboard size)
Power supply (matched to all components)
DVD/cd
Basically you build from the cpu up.
So
i5 2500K
socket 1155
P67 / Z68 motherboard
socket 1155, DDR3 ram, Sata conectors, ATX
Corsair vengeance ram
DDR3 1600Mhz
Sata hard drive
sata3 6Gb/s
graphics card
PCIe 16x
Case to fit all components.
mITX uITX ATX mATX FullATX
Power supply needs to be worked out using a PSU calculator depending on your system
DVD/cd needs to work on the motherboard SATA or IDE.