Cool, I didn't realize that there was a 128-bit version of the 9550.
Upgrading to a 9600 XT won't give you a huge boost. The only difference will be faster GPU clockspeed and faster gfx RAM. You already have the same core and you already have the 128-bit gfx RAM. If you want a bargain gfx upgrade, you should consider buying a used 9800 Pro from someone who's upgrading to even newer gfx. Have you tried overclocking your 9550 for now?
1 GB RAM is always nice, so keep the RAM on your wish list. But I think you'll find a faster gfx card would help you moreso. (And I wouldn't recommend upgrading to anything less than a 9800 Pro)