Xp Pro x64 is limited to 128 gig's of ram. Unless you are running a server you will never touch it. Of course, this will be determined by your motherboard, more or less, and again, unless you are running a server... Considering your applications, you wont see a difference in the same system configurations running in 64-bit OS vs 32-bit OS. There is, however, a massive difference in the speed between single/dual/quad.
In my opinion: unless you are going to profit from 3ds visualizations, you will likely never spend the money on the extra ram it would take to exploit x64's capability. Even more so, you may wind up limited on your software, and purchasing more to be compatible with your OS. It seems smarter to go ahead and get the ball rolling, considering that 64-bit is the new 32-bit, though. While many programs can run in a 64-bit environment, there are many that are simply not 64-bit "enabled", meaning that with a little work they can run within the 64-bit environment, but still 32-bit software in a 64-bit environment cannot utilize the memory advantages you are giving it. Think about the difference you saw when you first upgraded from 512 ram to 1gb. Now imagine what 4gb compared to 8gb+ is like, 64-bit applications in the right environment will leave your jaw dropped.
Spend your cash on a good quad core, and motherboard. 4gb of ram from new egg is $99, it cant be beat. You will be astounded at the performance improvements; I was using an AMD64 2.4ghz and 1gb ram, and have jumped the quad core band-wagon, with 8gb ram now, and my render times, not using the render farm, are 1/10th what they were, and I dont get pissed off with waiting for days for materials/shading/light cache/render/etc. But I dont play games; It's basically a cheap render workstation(compared with others on the market).
So to answer your question. You are the only one who decide the economical value of upgrading to a 64-bit environment. Right now, it seems, you may just want to upgrade some of your components until you are using 3d for profit, because new systems get expensive quick.