HD4870 512MB questions

I currently have the 8800GT 512MB card and would like to upgrade my card for better gaming. I have been recommended the HD 4870 512Mb card by a friend as I have a budget off approx £150 give or take a few £s (the cheaper the better ).

I have had a look around the web and have found 2 card which interest me but don't really trust the company. If i have to return the product for whatever reason I have to send it into Europe as my own cost.

Questions:

1. Has anybody used Pixmania? If so, can you tell me of your experience?
2. Is it a good card to buy as an upgrade after the 8800GT 512MB which fits into my budget?
3. Can you recommend any other suppliers which have very cheap cards?
4. Does anybody know of the lenght of the warranty I get with the card?
5. I dual boot between Vista and Fedora 10 KDE (Linux), would I have any issues with the drivers in Linux or with Compiz Fusion?
6. Are the graphics card brands important when purchasing a card?

MSI K9A2 Platinum motherboard
AMD Phenom 9750 2.4GHz Quad Core processor
Crucial 4GB 800MHz RAM
Nvidia GeForce 8800GT 512MB graphics card
3 x 500GB Seagate Barracuda HDs

Any help would be much appreciated.

Too expensive for the 4870, you searched for the 4850.

Are the brands names an important factor purchasing a card?