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would be better if we had any idea what your system is that you use and resolution ?
Aye, the 4890 is still a good card - it performs a bit better than a HD 5770/6770 and will generally play modern games well at 1080p res.
However, with a slow CPU like an athlon 64 and a low resolution (presumably 1280x1024) then in the vast majority of games the performance will be limited by the CPU, not the 4890.
May I ask what make and model number motherboard you are using?