For 1000 CAD you can get something nice :) but first...
E7500 @3ghz is old, you're gonna have some problems! I'm running a E8400 @4ghz and i get CPU limit at 50-60 FPS in Witcher 2. Heard that GTA 4 is even wors.
If you wanna Upgrade you also need new Ram (DDR3) i think you're still running on DDR2.
so
+300$ CAD for CPU :I7 -2600k
http://www.expc.ca/i-17115-Intel_Core_i7_2600K_Sandy_Bridge_3_4GHz.html
(210 if you want to save i7-2500K
http://www.expc.ca/i-17116-Intel_Core_i5_2500K_Sandy_Bridge_3_3GHz.html )
+150-300$ CAD Mainboard 1155. (If you buy K version of CPU, look that you get a Mainboard which can use Multiplier... was ist H or Z chipset? And also IF you Plan for the Future, look for a Bord with 2 PCI-E 16x Lanes, so you can use CF later on if you want to)
+100-150$ CAD RAM (DDR3-1600 (or 1333) 2x 2GB Kit.
And GPU
+300$ CAD AMD Radeon HD6950
http://www.expc.ca/i-16895-Gigabyte_Radeon_HD_6950.html
Total: 760 - 1050$ CAD.
Warning! do NOT Flash 6970 Bios on 6950 Card! because 6950 has other RAM! (Less RAM Voltage if i'm correct.) Use a Shadder Unlock mode, and Overclock GPU Manually (not RAM, only Core).
Also! There are New Versions of HD6950 which will NOT let you use Shader Mod oder Flash Bios! They are disabled on PCB. So you need to make sure that this Card will be able Shade mod (Needs Bios Switch, and... there are some Cards which have switch but no double Bios)
Hope you will find something you like.