A giant Zalman 120mm CPU cooling fan RARELY draws more than 0.5 Amps at 12VDC which is 6W.
http://zalman.co.kr/eng/product/view.asp?idx=145&code=005
High quality motherboards can support at least 1.0Amps at 12VDC some support 2.0Amps at 12VDC which is 12W and 24W respectively which is more than enough.
On a high-quality high-end Tyan S2882 Dual Opteron motherboard for example 2 fan headers are rated for 3.0Amps which is 36Watts! The rest are rated for 2.0Amps which corresponds to 24Watts of power.
That is more than enough power.
On a really cheap motherboard I might be concerned.
The ASUS A8 series support up to a total of 2.22A @ 12VDC or 26W.
I checked the manuals for the S2882 and the A8 series, both are online @ tyan.com and www.asus.com respectively.
When you use the 4pin molex power connector you lose the ability to throttle the fans unless you have a separate speed controller.