Nvidia. And you should refer by their model names. 6600lx vs 9550, I'd say the 6600lx could be better. I honestly don't keep up with the lower end cards (sub $100) as I consider them nothing more than DVI out-put add-on cards for business PCs. There are very few reviews of such cards.
Look at this chart for most current products:
http://www23.tomshardware.com/graphics.html
I never heard of a 6600lx, but I would guess it performs like a 7300GS (2nd green from the bottom) and that would be with a Core2 system... so take such scores and cut it by 50% will be a rough estimate of your performance.
So if you went to a 7800GS = then your frame rate would ber about 70fps rather than 16fps) But at that price range, ATI's 1950PRO AGP card is the very best - at $200 - which is the same or cheaper than 7600GT-AGP or 7800gs-AGP.
Read this:
http://www.tomshardware.com/2007/01/10/agp-platform-analysis
Hey, I just found something. the 6600XL is an OEM card made for only a few manufactures. It sits between a 6600GT and 6600(plain). So that chart above would put you about 25~30fps and its EASILY better than the 9550.
Also, video RAM doesn't always mean better. 256mb on a slow card will not make it fast.
The 4200-64mb card will always murder a 256mb GF 5200 or 5500 card.
You may enter a cross road. $200 for an upgrade card will give you a better gaming experince... and is fine if you're not planning on upgrading for over a year. But if your PC is feeling slow - that $200 could go towards a new system in which $150 gaming card would do just as fast... No more AGP systems are built. But with flight-sims, I think your system can last you for quite a while.