The problem with pre-built PCs, particularly little-known brands, is that you don't know what the PSU is, so you can't gauge how long the PC will last, or indeed if it will go bang, taking out half the components with it. My only other issue is the hard drive - I don't want to buy a PC with a Hitachi or Seagate hard drive, as I've had nothing but problems with them. Generally, the manufacturers will whatever is cheapest when they make their orders, as for them, that's all that matters.
Little niggly things are smaller issues, but still issues. Take that 6gb of RAM - the only way to get 6gb of RAM (expandable to 8gb of RAM) on a socket 775 motherboard is to use 3 sticks of 2gb. That will work fine, but there'll be a performance penalty, as really you'll want dual-channel, for which you'd need evenly matched pairs (for example 2x4gb, 2x2gb, 4x1gb or 2x2gb + 2x1gb). With only 4 RAM slots (which as far as I'm aware is the maximum a socket 775 board can have) you can't get to 6gb of RAM with a slot spare and retain dual-channel. It wouldn't be a problem on a socket 1366 board, as the CPUs in them can do triple-channel. Just looking at that tells you someone's not thinking about the system, and that makes you wonder about the rest of it, the bit the specs don't tell you.
Obviously with a home-built one, you know what you're putting in, so the PSU you buy is going to last, and if it does fail, it's not going to fry the rest of your system. It's a confidence thing really.
Most motherboards come with PCI ports still, though I personally wish they'd just kill off most legacy ports (or at least left them off the high-end stuff.
I'm not sure home-built PCs have been cheaper for a few years now, but that's because you don't buy the cheapest parts you can - you buy the parts that suit you.
Still, I've just totted-up an identical-spec system (minus the £3 keyboard & mouse, and with the brands I'd buy) and it came to £476 ex delivery.
Still, I've cooked up this (I'm just using Scan as I know what they have - most parts will be available from any decent etailer).
CPU:
AMD Phenom II X4 955 Black (£83.94)
CPU heatsink:
Coolermaster Hyper 212 Plus (yes the CPU comes with a heatsink, but it's a bit crap) (18.83)
Motherboard:
Gigabyte GA-880GM-UD2H (which has firewire around the back, and an internal port so you should be able to have a firewire port on the front of the case too) (£54.95)
RAM:
8GB (2x4GB) Corsair DDR3 XMS3 (4gb is enough for most decent systems, though ironically 6gb is the sweet-spot at the moment, but read above for the impracticalities of that) (£64.99)
Graphics card: S
apphire HD 6770 1GB (I guess as the Zoostorm system has a dedicated card you want decent gaming performance?) (£89.98)
Case:
Coolermaster Elite 334 (yes, it looks a bit naff, but
the case I love is £80) (£29.94)
PSU:
520W Seasonic M12II-520 (currently not in stock, but I'm sure it will be back soon) (£71.98)
Hard drive:
1TB Western Digital WD1001FALS Cavalier Black (£39.56)
DVD R/W:
Samsung SH-S222AB/BEBE (£14.66)
OS:
Windows 7 Home Premium 64-bit (£69.68)
Total: £519.68 ex del.
Over budget, but it's a nicely rounded system, you know exactly what you've got and can customise it for you. We could trim it down if necessary.
Just one note - the hard drive. I personally always have at least 2 hard drives - one smaller one (<500gb) for Windows & programs, and another larger one for documents, data, videos, music, whatever. It's just less messy if your Windows installation dies for some reason, and less things to remember to backup if you do reinstall. I'll recommend something along those lines if you want me to. You'll want an external drive for backups too. You can get an
ejectable drive bay which goes next to the DVD drive, and allows you to add or remove hard drives at will, even with the power on - perfect for backups, without the need for power cables and USB cables, though you'd need another hard drive for it too of course.