ipfw, pf and ALTQ on outbound traffic? (or: "The net is slow when I upload!")

[Jubal Kessler]

Greetings,

Is there a general how-to, or a set of coherent instructions, for 
shaping outbound traffic such that when I upload something over my 
asymmetric cable-modem pipe, doing so doesn't completely kill my Web 
browsing or any other attempts to use my Internet connection?

(To put it another way: When I max out my upstream, and my upstream is 
capped lower than my downstream, my downstream becomes useless and I am 
forced to wait until the upload finishes before I can resume using the 
downstream. This is a problem, and I'd like to solve it.)

I have looked at various ALTQ + pf setups on the Web, but I have one 
caveat. I use FreeBSD 6.4 on my home gateway, and it is also using the 
default natd server, which relies on an ipfw divert rule. I don't know 
if this matters, or if I need to switch from natd to a pf-based NAT setup.

Should I use *just* ipfw, or should I switch everything to pf (including 
NAT services) and go from there?

Thanks much,

Jubal