A few questions before switching

Hello Mac users.
I am seriously considering doing a complete switch from the pc to the mac. With complete I mean replacing my laptop, wireless network, backup harddrive and home computer with apple products (macbook air, airport extreme/expert, timecapsule and imac).

This might seem excessive, but after having had an iphone 3gs for around 8 months I am deeply in love with the whole eco system.

So far so good, considering what you have just read you might ask “well why not just switch? You obviously are in love with the products!” Well yes, but the thing is that I like an occasional game. Admittedly I have been falling more and more in love with the consoles lately, but I am still looking forward to half-life episode 3 and I would like to play this with mouse and keyboard.
Now, the steam mac client is currently in beta, so I am sure it will be ready to the release of episode 3, but will a 27” imac even be able to run this? The GPU (I think it’s a radeon 4850) in those seem kind of outdated, but maybe they perform well nonetheless? Or is a mac pro required to play anything?

Also, I am a frequent user of the ms office programs. Mostly Word and Excel. I know that you have iWork on the mac, but do they compare?
Last thing im wondering about is buying an outdated product. It seems it has been some time since they released new mac models, so maybe now is a stupid time to go spend money?
Another fast question – is the mobileme really worth the money? Almost 100$ (the price in my country, I guess it is a lot cheaper in the states) a year seems kind of costly for services that google can give me for free, but not having tried it I do not know.
Ok, so to summarize:

• Running Valve games on an imac
• Office/iWork
• Right time to buy
• Mobileme