Re: which card would be better
The HD 4870 is slightly faster than the GTX 260 in general. The 260 overclocks well, but then so does the HD 4870.
If you have an SLi board though, the GTX 260 might be the better choice as it adds an upgrade path later on.
AMD Phenom II X4 955 BE @ 4.0GHz + TU120E lapped - MSI 770-CD45 AM3 - 2x2GB OCZ DDR3 1333MHz - Sapphire HD 4870 - Samsung Spinpoint 500GB 7200RPM 16MB cache HDD - Tagan TG600-BZ Piperock - (Currently open test bed) - Windows Vista Home Premium 64bit.