Go to the Control panel then Administrative Tools then Computer Management on the left hand side at the bottom there is a setting called Disk Management click that and all your drives are in there - what does it say next and under the drive?
Please post on what Wolfey has said - there are normally only a few reasons for this happening - the refresh rate is set too high, the resolution is set too high, or the graphics drivers are out dated.
Come back with what Wolfey asked for and we will have more of an idea.
Only if you are dressed up like Darth Vader - :D
No seriously, you say that the router is in your room - are you using CABLE or ADSL - where is the modem based, in your room aswell, or at another location?
CROSSFIRE or SLI is overrated and not needed - of you can afford a card like the 4870x2 then get that one as they blow everything else out of the water!
You need to reinstall the drivers for the network adapter - if its a USB stick or an add in card then you need to uninstall the drivers VIA device manager and when you REBOOT they will be detected - or you can use the CD that came with the adapter/card
If it is what I think it is - you need a direct replacement as in the GPU that was in it - you need another one, have a look on the net for a 7500LE and unfortunately thats the only option I can see.
I did see this Scan 3XS X58 Core i7 | Desktop PCs | Reviews | Custom PC
And yes it is at Scan.co.uk
I am not sure if this is the one you mentioned earlier, but it seems ok for the money
How are you connecting to the router - USB or Ethernet?
It is odd that its dropping off - is it ADSL or CABLE?
It just seems like some of the settings are out.
If you have more than one stick of RAM in the PC then try taking one out - if it does the same take the other stick out.
It could also be the GPU (graphics) overheating through dust, so clean the inside of the PC with some compressed air.
That Siberian Water Cooler is rubbish, as for the expense - people charge alot for building PC's especially companies.
If you do not have the knowledge of building a PC then you are VERY limited in terms of choosing parts that are the BEST and then putting it all together.
Water cooling is only really needed if OC'ing.
620W is not alot they may tell you that its sufficient and it is JUST, if you are going to be looking at making it 'future proof' then the components that you add later like a second GPU, you will be well over on the power requirements.
You are so limited if you are not building and unfortunately you pay through the teeth for it - or bite the bullet and build.
Right - I dont want to sound like I am against all your opinions above, but if you are using the PC for gaming you are expecting the components to be at load most of the time, and you need to make provisions for that.
Any SATA drive will do - BUT I recommend that you get an enclosure for it to keep it protected and cool.
You can get a 500GB Drive for around £50 from Scan.co.uk
And the enclosures themselves are around £10-£15 - you can get external SATA drives from anywhere now but most just connect via USB or FIREWIRE - just make sure that on the back it has a SATA port
I had a quick look for you and found this XNE+8400S+TD26-PM8386 - 256MB Palit/XVision 8400GS PCI-E(x16), Mem800MHz GDDR2, GPU450MHz, 16Cores, DSub, DLDVI-I - Scan.co.uk