My system is also an older machine with a dual-core processor.
I added RAM (an even 8GB now), and over the last year moved from XP, to Win 7 32-bit, and now Win 7-64-bit -
and it runs GREAT!
X1 improved noticeably running 64-bit with 8GB RAM, and so far X2 looks like it is going to run just fine.
Instead of the $$ to buy a new machine, you should be able to get upgrade pricing to Windows 7, and have the funds to upgrade your audio interface and maybe add a 2nd drive for audio, sample, etc.
My X1 and X2 installation "share" all plugins folders, even though X2 has its own installation folder.
I did have to re-customize my plugin menus, but other than that both installations seem to coexist and work just fine! :)