The ssd will load everything noticeably faster and access times will be fast.
However if you have a really bad gpu a new one is just as big of an improvement as the ssd if you currently can't run games you like.
Kind of a loaded question.
Which is more important to you gaming or waiting 30 seconds for something to load?
If you already have a decent GPU like a GTS 250/9800GTX+ or 4850 then $150 isnt going to get you a significant upgrade, but if you are running a 4650 and want better gaming then $150 on a GPU will be a huge upgrade.
If you are using an old 250GB HDD and add an SSD it will improve boot times and load times on just about everything significantly, but if you are already running a few fast HDDs in RAID 0 it wont make quite as much of a difference.
It all depends, so post your specs and usage and we can be more helpful.
If you use it mostly for gaming and are using a resolution of 1680x1050 or greater upgrading to a 5770 with that $160 would provide a pretty big boost, if you arent using a large screen your 8800GT probably fares pretty well, so i would go for the SSD in that case.