heres one of the best bang for the buck enging options...take the head of your GSR..put it on a B20z bottom end..rebuild it with forged pistons, rods, etc...this combination is good for 180-190whp..you can get a B20z long block for about $900.00...and rebuild it and have the mods needed to make the b20z work with the b18c head...just some minor mods...you can read about this swap at http://www.automotiveforums.com/vbulletin/t48680.html
or www.crvtec.com
you cand get it all done for about 3k