Actually Purple Stank, that's pretty anecdotal advice right there.
You have no idea whether your "old HP mobo" is anything similar to this mobo.
The mobo says it has a PCI-E x16 slot, which will fit ALL current PCI-E discrete graphics cards.
I don't know of the situation in England but in the US recommending the GTX 550 Ti is suicide, as anandtech states:
"Indeed the GTX 550 Ti is faster than the 5770 - by around 7% - but then the GTX 550 Ti costs 36% more. At the other end of the spectrum is the 6850, which is 7% more expensive on average for 25% better performance. Even the GTX 460 768MB is going to gnaw at NVIDIA here so long as it’s still on the market; it’s 15% faster and yet it’s $20 cheaper. "
So the 5770 for basically the same performance, for a lot less money, the GTX 460 768mb for more performance, same price, or 6850 for a lot more performance, slightly more price.
I'm glad to hear that I wont need to dish out a lot of extra cash =D. Sabot, would those graphics card bottle necked by my cpu though?