If your willing to spend that much and make such a serious upgrade, then yes, moving to 64 bit will be a good improvement.
But to breath new life into your current system, I would think a 9800pro, 6600GT (agp) or equivalent would be a good start. Also, look into doubling your RAM.
-DFI Lanparty NF4 Ultra-D
-AMD 64 3000 Winnie
-eVga 6600GT (PCIe)
-Mushkin Dual Pack PC3200 (512 X2)
-80gb WD SATA HDD
-Creative Live! Series 5.1 surround sound w/subwoofer