WITCH CARD IS GOOD FOR MAC PRO 8 CORE & 3DS MAX

Hi sir/mem

my system is mac pro 8 core 3.0 with 12gb ram
in my mac pro 8800 graphic card is installed,
i,m using this system fro 3d rendering,
witch is not giving me good performance.
i,m using windows 7
i ,m working on 3ds studio max, Auto cad & photoshop
if u know what could be problem in my system please let me know what should i do to increase system speed.
witch graphic card u will suggest for my mac pro?
is ATI Radeon HD 5770 Graphics good for my need? or is any other nice one
but idont want spend lot of money on it.
same one pc i,m using in my office (mac pro 8 core 2.8 with 12gb ram) with 2600 model graphic card .this computer is working soooo good.i dont know what is problem with my home pc?
please solve my problem.
daily whole i spend with lot of tension because of this my home mac pro
thanks