Hell, the concept has existed in theory if not numbers since the pre AT IBM and Excel. 
It’s not a problem, it’s a feature! 
If you are referring to security overall, I’d say that Vista (64) is much more secure than Xp in that 64-bit operating systems are immune to all 32-bit viruses. This seems like the best virus protection available for anyone looking to be protected against 98% of every virus currently out there. Any viruses aimed at Vista 64 will almost certainly have to be written targeted solely at that 64-bit medium.
You WILL be moving to Vista eventually and you WILL be looking at a 64-bit system sooner than you think, so my advice would be to get a system that can run both and wait until service pack 2 before moving to Vista.
Windows XP can see and allocate up to 3 gigabytes of RAM with the 3 gig switch in the .ini file (and it performs well on every machine I have used it on (about ten or twenty), but you’ll likely need 4gigs installed for it to do so. Windows XP 64-bit will see much more.
I suspect Microsoft will cease supporting XP within the next three years. Also, MS has already anounced that Vista was simply a stepping stone to the next operating system that may very well be out by that time.
Vista is not immune to all 32-bit viruses. There are still macro viruses, MS-Word viruses, and so forth. And although they’re strictly speaking not viruses, I’d be surprised if it was able to block most of the Trojan horses, spyware, and so forth.
You can get an even greater benefit by switching to a non-Microsoft operating system, if virus protection matters more than compatibility. Almost nobody is writing viruses for Macs, Linux, FreeBSD, and so forth.
In fact, OS/2 Warp is darned-near virus free 
Why? Most other 32-bit apps run in Vista 64, even many low-levels ones. Why should viruses be any different?
Despite the number of folks claiming otherwise on the net, the /3GB flag is unrelated to the amount of physical memory in the machine – it affects the way that the virtual address space given to each running process is allocated. See Microsoft’s developer description here, especially the table about halfway down.
XP 64 is something of a lame duck, unfortunately; the driver model is different from Vista 64, and since XP 64 was never widely adopted, most folks aren’t willing to spend the effort to write drivers for it. (Of course, some folks don’t seem to want to write drivers for Vista 64 either, but they will eventually)
You don’t have to suspect; Microsoft publishes these dates. Mainstream support ends 4/14/2009, extended support (something businesses can buy, but most consumers can’t) ends 4/8/2014.
I strongly suggest XP, but so you can see both sides of the argument please see this ongoing thread:
Jim