i got the dual slot edition of the 8800GT for $150 during summer, and it doesnt seem like it keeps it as cool, my friend got the XFX one, and the heatsink was smaller, yet we got the same temps.
the 9800GT and 8800GT will perform the same, i think they got the same cores. the only difference is that the 9800GT is DX10.1 and has OpenGL 2.1 IIRC.