I play ET on a ASUS N50VN with a 15.4WXGA/LED display, DUO T5800 CPU, on-board sound & graphics and 4 Gb RAM laptop.
I can tell you first hand that a laptop LAGS big time compared to a gaming PC rig of the same cost.
The laptop just simply hasn't got the CPU, graphics and sound muscle that a PC has. It WILL lag. You'll need to downgrade a lot of visual and audio settings that you take for granted on a PC like the blood, atmosphere, sky, etc. with the Lap, but ASUS ION with DualCore series will run the games mentioned with no problem if you tweak down the lag and fps. The only reason you will need to tweak these down is if you start getting dropped by some servers for lag.
So why do I play with a Laptop? Same as you. I need a portable rig now and that's just the way it is. But if you get serious about gaming and competing then you'll have to go with a PC or die a lot.
I don't compete any more, but when I did I can tell you there's a definite difference when your rig lacks the poop to power the game at it's peak. I saw many a player over-spend on their rigs, but maximizing your game efficiency takes a certain amount of muscle that any thing slower will get you killed, cuz yer bullets hit a millisecond slower than your opponent otherwise; not to mention that what you see is a millisecond slower appearing on the screen than your opponent sees. I don't see that as a problem what-so-ever if you're playing-for-fun.
Just my 2¢
nicely put, and thanks. And I hate gaming on laptops