Okay, I'll go ahead and try for two out of three (maybe 2 1/2).
First. I'm assuming the eth0 and eth1 you mentioned are hostOS NICs. Getting the guest OS to use these is a breeze. You'll want to employ the "-net tap" option. The exact specifics of doing so are available in the man page however I will suggest a snippet that comes without even the slightest implication of usability.
qemu -net nic,macaddr=AA:AA:AA:AA:AA:AA -net tap,iface=tap0 \
-net nic,macaddr=BB:BB:BB:BB:BB:BB -net tap,iface=tap1 qemu.img
Notice that your specifiying different MACs for the interface (otherwise they'll both default to the same MAC) and while it may not be necessary to name the interfaces, I've found it to be so in the past (that's troubleshooting that you don't want to do).
Now, from there just to give you a heads up, qemu will expect an executable file in the /etc directory named `qemu-ifup` to be present. If what you have explained here is the sum total of your Qemu networking, the contents of the qemu-ifup file may look something like this:
/sbin/ifconfig eth0 down
/sbin/ifconfig eth0 0.0.0.0 promisc up
/sbin/ifconfig tap0 0.0.0.0 promisc up
/sbin/ifconfig eth1 down
/sbin/ifconfig eth1 0.0.0.0 promisc up
/sbin/ifconfig tap1 0.0.0.0 promisc up
/sbin/brctl addbr br0
/sbin/brctl stp br0 off
/sbin/brctl setfd br0 1
/sbin/brctl sethello br0 1
/sbin/brctl addif br0 eth0
/sbin/brctl addif br0 tap0
/sbin/brctl addbr br1
/sbin/brctl stp br1 off
/sbin/brctl setfd br1 1
/sbin/brctl sethello br1 1
/sbin/brctl addif br1 eth1
/sbin/brctl addif br1 tap1
/sbin/ifconfig br0 188.8.131.52 up
/sbin/ifconfig br1 192.168.0.201 up
But wait there's more. So far we've discussed which command line flags to use as well as teh cretion and configuration of a file, BUT we also need to make adjustments to the kernel. You'll need to compile and load the kernel modules for briding (bridge.ko) and for the tun/tap interface (tun.ko).
We are almost there my friend (this sounds like a lot but it's really 15-20 minutes worth of "work"). There are many different ways to do this next part but I'll tell yo uwaht I think is best (since you asked). The tun module will create a dynamic device called /dev/net/tun which qemu will need access to to create the device. for this reason most people say run qemu sudoed 'cause root has access to everything. NOT A BIG FAN OF THIS IDEA. What I did instead was edit the /etc/udev/rules.d/udev.rules to dynamically assign ownership of /dev/net/tun to Owner:root Group:net. Then I just created the net group and added myself to it. Also, /etc/qemu.ifup should have the same ownership levels. Knock out these three things and you SHOULD (no warranties!!) have what it is you're looking for. It's really not as complicated as it looks (20 minutes TOPS).
For sharing files between guest and host OS' there of course is the -smb option which requires you to have smb installed on the host OS. Again, not my preferred option. What I do instead when moving files back and forth is just scp them. 10.0.2.2 is the host os and you can scp (on a host only connection) from your guest os to your host OS no problem. Your max DTR is about 200k-300k so for larger files beprepared to wait a while BUT that has nothing to do with the scp option. If for some strange reason you prefer scp'ing from the host to the guest over vice versa, you can use the -redir option to allow connections in that direction.
That's my promised two out of three, now as far as the modem thing is concerned, who the heck still has a modem?? J/K I know a few thrid world countries still use them regularly (potable water first, then high speed internet). The bridging trick from the first answer should work once the modem is establed on ppp0 or whatever interface it's going to be. Now I know I said the other stuff isn't guarenteed to work, but this really isn't as this is something I've never actually done buffer (just shooting from the hip). Good luck.
P.S. Just to recap in case you got lost, you will need:
Qemu Flag: -net nic -net tap
Kernel Module: bridge.ko tun.ko
Permissions: /etc/demu-ifup /dev/net/tun
Software: scp, WinSCP
[everything from 1)]