Hey mate,
I'm in exactly the same position as you currently with an Asus W3J, just slightly lower spec. Core Duo 2.0Ghz, x1600, 80gb hdd, 2gb Ram. I have similar desires in gaming too.
I'm keen to make the switch to mac but I'm finding it hard to justify at the moment with the current offering given the following (in Australia with edu discount + applecare):
-The Asus is 1.5-2yrs old now and at the time cost $2400AUD (1900 US)
-MacbookPro line starts at $3500 (2750 US) which is just too expensive and specs arn't significantly above the Asus.
-Macbook Alu top of line is about $2700 (2100 US) for the similar specs (notably GPU)
-White Macbook is $1800 (1400 US)
So really it's not looking like it makes sense to move to any of those given the cost and the negligible difference from what we already have. With that in mind I've been thinking of an iMac to get more bang for my buck,
-iMac 24" 2.66 base - $2500 (1950 US) - Better CPU, great screen, same GPU
-iMac 24" 2.93 w/ ATI 4850 - $3350 (2600 US) - Expensive but better CPU, great screen, much better GPU
So what I'm taking from all this, and maybe this will help you too, is that gaming on the mac is simply not feasible for a reasonable price. That's fair enough and people will tell you that straight away, so perhaps move away from the idea of intense gpu and focus on the other stuff you do. Invest in a console (like me) or scrap the mac idea and get a pc. My friend is thinking of an i7 build about $2000 (1500 US) which is sounding pretty amazing.
So with that I'm thinking of just a generally ok all around computer, and from the prices I'm leaning towards the 2.66 iMac for some quality screen and performance action, or a cheaper white Macbook for portability but the same performance as what I'm used to and try OSX. All the other options are too expensive for what they offer to me.
Problem is I want to upgrade in specs, but want to stick to laptops since I got this Asus. Realistically I'm not going to swap between them if I get an iMac because I'll tend to stick to the newer machine for everything. Decisions Decisions.
Other considerations though:
-Sell the Asus for a bit more cash to upgrade to a more powerful mac
-Wait for snow leopard announcement (6.5hours!) and maybe hear about grand central. If they announce something amazing with general processing gpu stuff then that could push to spend bigger bucks on a quality gpu
-In the same line of thought they might upgrade all their gpu's across the range to take advantage of new snow leopard features. The white macbook is cheaper and better than the entry level aluminum, so that also feels like there could be a bump there.
Hope that gives some new ideas for you. Let me know if you decide something to maybe help me out with my decision =D