I have the same dilema - I just ordered the m1330 and then I noticed I could get a better graphics card in the 1520 for less.
I've done a lot of research and I'm still confused.
This site clearly shows a massive difference between the two cards:http://www.notebookcheck.net/Mobile-...ist.844.0.html
The 8600GT kills the 8400GS with more than double the 3Dmark06 score.
However, this site:http://www.laptopmag.com/Review/Dell-Inspiron-1520.htm
Although showing a superior 3Dmark for the 1520 with 256mb 8600GT reports poorer gameplay results with Fear playing at only 12fps on full details compared to 15fps on the m1330 with 128mb 8400.
I will venture to say in practice I would imagine the 1520 to have superior gaming performance and the Fear test was just a one-off.
The problem I have is the features. The m1330 has the nicer screen (although lower-res it is back-lit by white led so it's brighter, thiner and consumes less power), it has HDMI out, longer battery life (1/3 more when doing a draining task and even more with light use), MUCH lighter (4-4.5lbs vs 6-6.5lbs).
My brain is telling me pick the 1520 you need the graphics power. My heart tells me pick the more stylish m1330 and play your games on a HDTV.
I watched a bunch of videos of the m1330 playing games on youtube and it seems to hold up well with current and older games like Half-life 2 and fear but I am worried about games like Crysis and BioShock. I know that DirectX 10 games will run 20%+ faster in DX10 mode so I know both laptops will improve in performance when the DX10 games start rolling out compared to users with DX9 cards running the games in DX9 mode but I don't want to have a choppy frame rate or have to turn off all the features that make the new games so breath-taking.
Maybe I'm expecting too much and should just shell out for a xps m1730?
I've already placed my order for the m1330 so I only have a few days to change my mind. Let me know what you decide.