Linux is MUCH more reliable and secure by its design than Windows. You said you have a Pentium D -- I'm running an Acer Power series with the same processor. I'm running with 1 Gb of RAM, and OpenSuse 10.2 also. OpenSuse will run pretty well on 512Mb, and I got it to go on an old 633Mhz Celeron, but it was really slow.
For drivers: The Pentium D type machines have been around long enough that you shouldn't have too much trouble getting things going "out of the box" so to speak. Suse found all the hardware on my system without any trouble, but if you have a problem don't be afraid to ask for help. Many people have trouble with wireless networking at first, but standard ethernet connections usually aren't a problem.
Maintainence is pretty simple for most Linux systems, and Linux has its own firewall software which in Suse is set up through Yast. Antivirus isn't really needed unless you've got some Windows machines on a home or office network. If that's the case, there are anti virus programs to do that.
As far as whether to go with 32 or 64 bit, I went with 32 bit because it seemed easier when I did my last install. There were still problems with multimedia and some other issues with 64 bit back then, and I don't know whether they have been fixed yet or not. I would recommend going with the 32 bit version, especially if you are just starting out with Linux. Its simpler and will give you time to get used to the Linux way of doing things. You can always upgrade later, when you're ready.
Hope this helps.....