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.
i would modify your signature to 5 lines max before one of the moderators here catch you. just giving you the heads up . i got a warning for it myself one