Actually I know 2 people who bought their LapTops with XP64 running and they are great.
XP64 does not NEED more RAM... It is written to make more use of the RAM, and so in theory, it should run sweet as a nut with 2GB over 1GB, but to be honest, I found this to be untrue on my P4... or at least I saw no difference at all in performance comparing XP Pro 32 & 64 with both 1GB and 2GB of RAM.
AntiVirus / Firewalls on XP64?
I use NOD32, its perfect and as has already been mentioned, AVG also runs great and I have heard that AVAST also runs perfectly well to, but I have not tried it myself.
As for a firewall, I cannot comment as I use a hardware one, but the built-in one is very capable.
As for speed, this is something that has been bugging the hell out of me, but the first time I tried XP64, I ran it on my MSI Neo2 Platinum, with a Winchester3200 & 1GB RAM... At the time I could not run the USB Modem ( BT Voyager 100 ) and the IDE Card ( I like lots of HDs ) and so I could not get it fully running right, and I found it to be much, much quicker than plaing old XP32... So, I decided that I would upgrade to XP64 ASAP, and in the mean time, I have gone to a router, and found drivers for the IDE Card ( ITE 8212 ) and my printer etc, and I went back to XP64 and found no speed increase at all???
I put the trial back on and it was quicker???
Hows that for annoying!
Anyway, bottom line is that XP64 and XP32 for 99% of what you do with it, is no different in terms of speed, whether for gaming or for general apps.
I myself have only found a few thigns that are not fully working.
PageDeFrag
Partition Magic 8 - Works fine, but if it needs to restart to alter anything, it wont work, so resizing partition that dont have a page file on, are fine, but you cannot shrink C: only make it bigger, but other than that its ok
O&O Defrag - There is a Beta of v8 and it works, but there is a few inconsistencies when compared to the 32Bit version, other than that it works 100% fine.
Thats pretty much it really, all my games work just fine, and yes, HL2 and FarCry have 64Bit updates but to be honest, HL2 takes long enough as it is to install and update without having to wait another day or two for it to alos do the 64 bit stuff too! - I hate steam.