"upgrading" GPU on mac Pro 1,1

recently, video card (ati radeon 3870) on mac pro 1,1 died. have been searching replacement, , not having luck. however, i've found 2 candidates , need advice on pick.

first nvidia 8800gt 512mb, $200
second ati radeon 5770 1gb, $266

price performance wise, 5770 wins sure. however, i'm not sure work in computer if don't have snow leopard installed (before computer became inoperable, had 10.5.8 installed on it). have purchased 1 card (gt 120) didn't work have 1,1 mac pro , looking avoid future mistakes. know have upgrade snow leopard drivers ati card, able @ least boot computer , install snow leopard on if buy ati, possibly in limited resolution or color mode?

really, sucks these options upgrading. anecdotal evidence amazon customer reviews says 5770 work on 1,1 mac pro, have no idea if of users had snow leopard installed or not. appreciated, thank much.

mac pro 1,1
osx 10.5.8
6gb ram
quad-core 2.66ghz
no video card

 

if these mac models, go 5770, work in mac pro 1,1 had 1 in mac pro 1,1 year, fantastic card, , worth money imo.

noticed mentioned 10.5.8 still on leopard or did upgrade snow leopard or lion? know 5770 work in snow leopard or lion, not sure leopard though.

 

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