ATI Radeon X1900 XT 512 MB: Problems

[開発者ἀνδρεία]

A lot of games don't even start with it, and if the games start, they go crash.
What's wrong with it? These games don't work:
-A virus named Tom (Crash when opened)
-Anomaly Warzone Earth (Same Thing)
-Assassin's Creed Brotherhood (only the sound start)
-Assassin's Creed II (don't even start)
-Awesomenauts (same thing)
-Batman Arkham City GOTY (same thing)
-Borderlands 2 (it start, but the video is bugged or something else)
-Brutal Legend (don't start)
-Fez (same thing)
-Left4Dead (same thing)
-Left4Dead2 (same thing another time)
-Limbo (you know)
-Mark of the Ninja (it start, but the video is black)
-Portal 2 (don't start)
-Rocketbirds (don't start)
-Rogue Legacy (start with a crash)
-The showdown effect (start and close)
-SM Civilization V (start but it don't have any text)
-Trine 2 (Unplayable for lag)

But it work perfectly with those few games:
-Ace of Spades
-Amnesia the dark descendent
-Bastion
-Botanicula
-Evoland
-FTL Faster Than Light
-Garry's mod
-McPixel
-SM Civilization IV
-Super Meat Boy

The mac is a Mac Pro
Processor: 2 x 3 Ghz Quad-Core Intel Xeon
Memory: 14 GB 667 MHz DDR2 FB-DIMM
Graphic: ATI Radeon X1900 XT 512 MB
Software: Mac OS X Lion 10.7.5 (11G63)
Can someone helps?
P.S sorry for the english, i'm italian.

[Chas.]

There's nothing wrong, your GPU is 7 years old and grossly under powered. You will not be able to play any modern game on it.

Sorry.

[Lantern]

get a new computer

[NotAGuarantee]

Get a new Imac 2013 if you still like Macs. Or just get the best Macbook Pro!

[Riwels]

You should update your OS and your graphics card. You can get OS X Mavericks for free in the store.

For a graphic card, the current iMacs comes with NVIDIA 7XX, so I think a GTX760 or GTX780 are a good choice.