i'm trying to follow the logic here. 10.2 is slow on your old mac, so you're going to abandon X altogether and buy a new, fast, pc?
what about the option of buying a new, fast, mac? X.2 screams on my dual 867. $1700 + $100 to upgrade to ATI Radeon 9000. multitasking, as stated, is where X shines.
also, have you tried tweaking the OS to make it a bit faster?
.: install a simple, sharp cornered theme
.: put minimize to scale
.: install windowshade and use that instead of minimizing
.: use chimera instead of IE ^_~
.: adding quality RAM
.: turn off dock effects (magification, hiding, etc.)
and other little tweaks you can find at resexcellence.com.
it helped my old imac quite a bit, and it's helping this G4 450 that i'm using quite a bit as well.