yeah there is a point of diminishing returns if you really wanna upgrade your fx.
i think the most effect upgrade would be a T9300 or T8300 or even a X7800 (all of which can had for under $200 on ebay if you have a little patience)
and pair that CPU upgrade with a nice SSD and you are looking at a very impressive notebook. You can pick up an ocz 60gb for $130 so its not very expensive.
I found that using certain video drivers would gain around 20fps in source games (which i play mostly). the one on the nvidia site is not very good. this thread
is a great resource in finding the best driver for your games.
and like warlord said the gpu can be overclocked pretty high. I have about a 15% overclock on the card (core,shader and mem) so its helped gain around 5-10fps. I keep it on a chill mat so its stays nice and cool (you should too).
my gateway fx 6860
T8300 @ 2.4ghz
4GB DDR2 533
WD250BEKT 7200RPM hdd
Win7 x64 Ultimate
looking to get an SSD but a little strapped for cash atm.
and it plays all of my source games flawlessly. perfect for when I visit my parents or go on trips.