now on the same cpu i wanna do designing & want to use software like 3ds max 2008, unigraphics nx4 & adobe photshop cs2 & other for 3d modelling.
should i upgrade to Windows XP Pro 64 bit edition?
will this upgrade help me in performance of these softwares.. i hav ram of 1.5 gb ddr2 500mhz.

If you think it's worth the money or the effort then yeah I'd say you'll definitely get a performance boost but you'll have to trade it off with how long/how much it cost to upgrade.

A 64-bit processor means it has buses and registers that are 64bits wide. A register that is 64 bits wide can store data or instructions that is 64 bits long compared to the 32 bits processors. A 64 bit bus is capable of transferring 64 bits of data or instructions per cycle.

Upgrading to 64 bit OS will not give you noticeable ferformance improvement.