If you need to improve your graphics cards temps then you could get an aftermarket air cooler for around £40
Alternatively if you want the best temps then you could opt for a custom watercooling loop, however this option will cost considerably more (roughly £200), needs alot of research in order to set it up and maintain it.
Ahh okay I thought that. I thought there might have been a way to remove the CPU block and then purchase a GPU block to add to it. I'm having some trouble with Skyrim as it seems to lag after around an hour of play, and the only reason I think of this happening is due to heat.
Ah right ok, well to check your theory you could download Hardware Monitor (freeware), open the program then play Skyrim for an hour, close Skyrim and check the maximum temps recorded by hardware monitor.
If any of the temps are North of 90C then you've got a problem
Unfortunately you've (or whoever built the PC) has made a classic mistake of skimping on the PSU
Almost all modern components use the +12V rail(s), your power supply has a 13A and a 14A rail, lets be very kind and say the PSU is able to supply both rails fully at the same time, that leaves you with 27A * 12 = 324W
nVidia recommend 400W for the 550Ti + sub system
I'm not saying the cause of your problems is the power supply, but I wouldn't trust it at all.
Check the temps as they could be causing the issue, bit of troubleshooting needed
I'd recommend replacing the PSU anyway, even if it isn't the cause of the problem, cheap power supplies don't have the same protection as good quality power supplies, so if a cheap PSU fails it could kill components that its connected too.
The PSU's I mentioned are made of much higher quality components, which result in the PSU being able to output its rated wattage continuously, also good quality PSU's have protection like OCP (over current protection) UCP (under current protection), OTP (over temperature protection) ETC... they are all designed to protect the PSU and the hardware its connected too. Also tend to have less ripple and variation on the rails resulting in better stability and safety.
No problem, the 3 PSU's I linked to before are just examples of quality PSU's that would comfortably power your system, there are many alternatives depending on your budget, generally anything from Corsair, Enermax, Antec, OCZ, PC power and cooling, XFX are good quality.
Once you've found a PSU you like the look of why not read a review, Jonnyguru test PSU's to their limit to see what there made of
there are things like the H60 specifically for graphics card cooling, I've seen a couple of pny cards that come with it already attached, or you can even modify one like you first said, heres a handy rip guide for that, http://www.overclock.net/t/754545/ultimate-h50-mod-guid...
but I would see what a decent Psu does for you first,