Advise on a new graphics card?

[Sky999]

My current graphics card is a ATI Radeon HD 3870.

I am looking to buy a new graphics card for SH5.

I am willing to spend up to £150 - 200 on a new one.

Can anyone advise me on what graphics card would be the best for SH5, would be superior than my current card and is in my price range? Is it just a matter of replacing the graphics card and then my PC will be top notch?

I will most likely hold back to nearer the release date in case a new, better card is released.

My specs are below.

Manufacturer - Dell
Model - XPS XPS_420
Rating: 5.9
Proccessor: Intel(R) Core(TM)2Quad CPU Q8200 @ 2.33GHz 2.33GHz
Memory (RAM) -4.00 GB
System type: 32-bit operating system
ATI Radeon HD 3870

PS: If there is anything else I can buy or upgrade to that will really increase my performance, let me know. I've heard people say upgrading from Vista to Windows 7 isn't really worth it.

- Sky999