Had this problem with tiger, is continuing with leopard.
on a G5 iMac

Using firefox, also does this with safari.

Not much faster than dial-up

Anything we can tweak in either the OS or the browser?

We have charter cable and do have tv and phone also,
but I just tried to download a 590kb file and it took
over 2 minutes!! and that was without the tv or phone
in use.

We've complained to Charter but to no avail.

Actually the Internet speed is governed by your ISP.
But in saying that, there are a number of local (your end) factors that can cause this

Wireless
Router configuration
Network setup
Virus and Spyware
Faulty hardware

How is your system configured:
Is your ethernet cable plugged directly from your computer to the modem
Or are you going through a router
Are there other computers on your network also connecting to the Internet

Sorry about all the questions - but you also be getting an understanding of the cause of this slowness from these questions.

Where are you downloading the file from? Usually the bottleneck is not your internet connection but some other point between you and the server.

And yes.. What is you network like? Have you tried with some other computer?

Not wireless, cable.

Router, one computer on it.
Same router we've had for several years,
this slow down problem has just been evident
but getting steadily worse over last several months.

Have tried other computers with same result.
Both PC and Mac, both of which worked fine
previously even when 3 computers on that same router
at the same time, now just this one.

Same anti virus in use for awhile, same as
used on the other mac tried. Worked fine before.

Both macs recently tried are relatively new
G5 iMacs.

We've been having similar problems with the tv
also - can hardly watch HD cuz the picture breaks
up then both the picture and the sound stalls.

Going to non HD it's better but still flickers on occasion.

Was having same problem with Tiger, both Macs,
now we have Leopard but no change with the Leopard.
Worked OK for several years with Tiger, and with
2 Macs plus 1 PC on it. Now it's worse with just 1
computer on it.

I believe the ISP is splitting the bandwidth up too much.

I work away from home and was in an apartment in a small
town recently and talked to the installer for the building.
I got phone + internet + tv and he said he re-allocated more
bandwidth to me cuz I had more things than others in the
building, he said three's only so much bandwidth coming
into a building or, in our case here, into our subdivision.

The problems are more noticeable during peak hours and
especially over the past holiday weekends with everyone
home.

Doesn't matter if I'm actually trying to download something
or just open up the home page.

Well to be fully confirmed it's not you, so as to complain to ISP
You would need to take your computer physically to another location (with the same server and speed account)

I think in this case I'd fault to your local phone line company first (to check the line)
And then back to the ISP

I agree that in these larger buildings with lots of users on the same bandwidth line, that the speed can be significantly be slowed.
Difficult for the ISP to fix in this case.

We're hooked to cable, not to any phone land line.

i did a search for something like Download Speed problems
with charter and came up with numerous blogs FULL of
complaints of the same thing from all over the country -
people getting very slow download speeds both on internet
and on tv.

Problem is they dont provide enuf bandwidth when the bring
it out to an area, whether it's a building or a subdivision.
they split whatever the main feed is into a given area up among all
customers in that area or bldg, so the more users that share the
same bandwidth the slower it goes for all of them.

One of the iMacs we checked with was our daughter's. Same problem
But at her place, in town, it downloads much faster.
We're in a subdivision in a small town about 6 miles from the major
town in the area where all the feeds come from.

We're paying for the middle download speed, forget the #, but our
downloads this weekend were nearly as slow as dialup. Our download
manager this afternoon showed we were only downloading at from
10 to 25 kbps, not mbps! And I think we're paying for something like 20 mbps!

I'd be upset too.
Cable of course - sorry

Anyway how many times have approached your providor, also are others there in the building having the exact same issue as you
Many people protesting is better than one !