Avast:
avast! Antivirus and Windows x64 Edition
Comodo:
Comodo™ Free Firewall Software Download
Both say they support Vista 64, though you'll need clean installs of the latest versions. But you'll be having to do clean installs anyhow: Vista 64 isn't upgradeable from other versions of Windows. So it's clean or nothing.
One thing I will say: Your post says 4GB of RAM: There is a hot fix KB929777 which you'll need to install prior to installing all of it.
(1) Load the OS with a stick or two.
(2) Leave the LAN cable unplugged until *AFTER* you have installed local (chipset, etc) drivers.
(3) If you're *really* prepared, you'd have your Drivers for your Vid card and peripherals saved on a CD/DVD. Install those before hooking up your LAN cable as well.
(4)Why???? Because Vista will automatically configure the network, auto-register, and automatically begin downloading and automatically install all of it's updates. You can do it in a different order, but In My Humble Opinion it's best to have your local stuff done first. Hard to cleanly install a driver when the OS installs an update and reboots in the middle.
Regarding "Loads Of Problems" in Vista 64?? I've been running V64 for half the year now, and have yet to have more than passing trouble with anything. And nothing I couldn't solve by poking through the manual and applying a little common sense. I had to upgrade my Anti Virus suite, and I got tired of waiting for the monkeys at Creative to write a 64 bit driver for their sound cards... Besides that, I've had no device troubles. Though I am aware there were a few RAID chipset/driver combinations which were bothersome. Stick to bigger name brands for your components and you really shouldn't have any trouble.
Even the install went problem free, tho I don't trust windows to format drives for me. And I *N E V E R* flash a Bios from within an OS. Out of long habit I drag the FLOPPY drive out of the drawer, boot to a C prompt, and do that stuff the old fashioned way.
There's some annoyances - User Account Control constantly asking for authority being the obvious. The other is Vista 64's pathological hatred for unsigned/expired digital certificates - Speedfan and CoreTemp didn't make the cut here. After that - There's basically just the occasional
"Why did Microsoft decide to move that..." game of Hide the Sausage.
"Live With It" stuff: Your HDD will be pretty busy for a while - By default the OS indexes everything for faster searching. So near constant hard drive churning was a fact of life for a while. But I really like the new, fast, and usable Search.
(Search results as fast as you can type) It's also VERY aggressive about using system resources. People who are used to their XP boxes idling along at sub~500MB of RAM usage *really* freak out over this one. Vista pre-caches all the stuff it thinks you may use into active memory for faster launching. So "Usage" goes from less than 500 to 1.2GB or a little more. But know that if an active program asks for resource, it is surrendered just as if the RAM were empty. A lot of people seem to not (want to!?!?) understand this, and scream 'Bloated Pig'. <shrug>
(Should I be?) 
