Until now, the iPhone was king thanks to its use of OpenGL ES 2.0 graphics, but with the added raw power of Maemo 5 the N900 certainly has the muscle to beat Apple in this regard. Despite having this advantage, games don't seem to be one of Nokia's primary objectives (they come 18th in a list of 20 technical specs on Nokia's website while the free games you get with the phone are Bounce, Chess and Mahjong. Zzzzz).
A key thing to remember is if the N900 has access to Nokia's Ovi Store, then developers will be able to produce games for it in the same way they do for the iPhone. Games played on the N900 would look better, though, since the N900 and all future Maemo devices come with 3D graphics acceleration hardware as standard and the 800x480 screen resolution will give a crisper image than even a PSP. This also means a broader range of ported games can remain in their native resolutions without having to downsize.
The N900 also comes with a dual PAL NTSC TV Out so you could play games on your television (this will sweeten the deal for movie fans in particular). A lot of Symbian operating systems use Flash Lite for games whereas the web browser for the N900 has full support for Flash 9.4, allowing users to play bigger and better Flash games. Meanwhile the cross-platform development QT widget toolkit makes it easy to release software on various platforms simultaneously and this will increase input from more third-party publishers.
Basically the potential is there - it's just a case of seeing whether Nokia act on it or choose to focus primarily on marketing the N900 as an all-in-one pocket PC. Even if there isn't any Ovi Store support when it launches, the N900 will still have built-in support for Ovi Maps, Ovi Files, Ovi Contacts, Ovi Share and other Ovi services. Maemo devices always carry some sort of software distribution system in the form of software catalogues so even if Ovi Store isn't included, chances are something similar will be.
The iPhone 3GS sounded pretty cool until everyone got a whiff of the N900 and saw that, actually, there was plenty of room for improvement. On technical forums the latter is overwhelmingly the current favourite, but at this stage a lot of the discussion is hypothetical since the N900 has only been out in America for a couple of weeks and won't get its full release until October 19th. Once everyone's had a chance to play with it and compare it more directly to other 3G phones, then we'll see if it lives up to the hype.
Resources:
http://www.fonearena.com/blog/2009/08/28/apple-iphone-3gs-vs-nokia-n900.html/comment-page-1
http://www.pocketgamer.co.uk/r/Various/Nokia+N900/feature.asp?c=15218
http://www.ovigaming.com/news/item/10390_What_the_N900_and_Maemo_5_coul.php