9700 vs 9800 perfWell, some would say you can nearly double your performance... The fact of the matter is, on paper it would seem that way, but you always have overhead on any video card (some greater than others, due to management of registers in the GPU and memory management of the onboard memory). That said, here's the low down.
9700 on paper is half the machine of the 9800.
9700 - 4/2 pixel/vertex engine pipe lines (Dx9 effects)
9700 - 128 bit memory to GPU interface (wider is always better)
9700 - 64/128 MB Configurations (you mentioned having the 128 MB)
9700 - My 3dmark 05 score ~1300 (Pre cat 5.7 drivers, then ~1700 after) **
9800 - 8/4 pixel/vertex engine pipelines (Dx9 effects)
9800 - 256 bit memory to GPU interface (wider path is always better)
9800 - 256 MB Configuration (bigger graphics memory helps future generation game developers.
9800 - My 3dmark 05 score was ~2900 out of the box **
** Numbers can be deceiving but it was clearly noticeable watching the test loop run.
Don't ask about the the clock speed's being different, how the 9700 is higher than the 9800... That'll take another long paragraph to explain, and afterwoods you'll see that they are meaningless, as apples can't be compared to oranges.
Here's my real world take on weather or not you should do the upgrade: Do you plan on using this laptop for at least another year, maybe longer to do things like play new games when they come out? Do you enjoy turning things up like graphic detail level without the big cost of FPS?
I upgraded my system last summer. Big difference. CSS and HL2 were the games I was into, which I can say they looked nice on my 9700. Post 9800 install, I launched those same games and was blown away at the FPS I was getting (same detail settings I used on the 9700). I then decided to stick with the same resolution but turn on things like AA (2x) and AF (4x) and noticed the game still played well without the high cost of FPS.
Today I am probably one of a few on the (Pentium 4 Notebook) forum who play Oblivion on my Pentium 4 based laptop (being careful not to get into a rant about how the XPS2's and up can easily, i know they can without problems). I have high praises with this system as I have an IBM laptop at work (which has a mobility x600 which is ~ what a 9700 was) and the game isn't tolerable on my work laptop. Home, I get a constant 45-60 fps at greater resolution and spend countless hours running into and out of the gates of Oblivion. I feel as though being able to play that game (if you haven't seen/heard of it, go take a look at a review or two... amazing) made my $320 investment last summer well worth it as i am able to enjoy it on pretty much any game out there right now (yeah, Quake 4 also played well).
I guess the bottom line for you is, do you want to spend the money?