yes, the mac vs. pc is the biggest issue. However, I'd take the G4 out of the picture. Yes, it is a very solid lappy but the processor is seriously outdated. Apple needs to realize that the core is primitive and need to stop increasing the clock speed. The Pentium M processor will give you faster performance but also 4-5 hr battery life. You can get the same video card (128mb 9700; if you're looking at the higher-end apple) and a faster 7K60 drive all for a cheaper price. I'm guessing you'd like the widescreen form factor so here's your top choice: the mitac 8050. The acer aspire 2025 is nicer looking but more expensive. IF you buy the acer, you'll have to take out the slower Hdd; the price will still be cheaper than the G4.
If standard screen isn't an issue then check out the CL56 (forum favorite) or the aopen 1557.
mitac 8050: rjtech.com, mayhem G2 @abs.com, hypersonic-pc.com
Cl56: rjtech.com, ibuypower.com, internetishop.com
aopen 1557G: ibuypower.com