I've been tasked with migrating our company internet connection where we are unable to retain our existing public IP subnet. The last time I did this it was a bit of a mess as we were moving buildings too, and ended up having to run duplicates of our servers at both sites for a short period until DNS propagation was pretty much complete (a few annoying ISPs appear to ignore TTL settings and fix their caching, so reducing the TTL down to something like 30 mins and then switching everything doesn't always result in smooth migration).
What I was hoping to do this time around was something a little more elegant - move everything to the new connection (which luckily this time shouldn't be too hard as most of it is NAT'd through a PIX, so it should be reasonably easy to just reconfigure the PIX mappings and external IP) but leave a server connected to the old line running an application proxy/firewall for incoming DNS, HTTP, HTTPS, and SMTP.
What I'm after is recommendations on a proxy/firewall that can do this without needing much in the way of hardware (although I can probably commandeer an XP2400 with 512MB DDR for the job, most of the spare PCs here are Celeron 300 with 160MB SDR), simple to configure, and unfortunately above all it has to be free (or at least extremely cheap) as my department budget is already gone and as this is a temporary requirement (at most probably to run for 2 weeks) my boss is reluctant to provide funding (despite the migration resulting in savings of £5k per annum). I guess I'm looking at a Linux based solution that's pretty much already rolled together, and just requires the public and private IPs configuring for each service to be proxied, even though I'm a Win admin I'm happy to turn my hand to messing around in Linux from time to time but I'm a complete novice.
So, any recommendations?