The Alternate Configurator is pretty good indeed, especially regarding compatibility of Mainboard/CPU and power requirement for the PSU. Their service is mostly okay.
In case some price comparison website favours Mindfactory: I've had a support nightmare with them, with many very unprofessional (sometimes directly insulting) mails received over several months and them refusing to replace components on warranty. It's a few years ago now but I'm avoiding them ever since.
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