Upgrade options for Acer x5900

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Dear everyone,

Here is my current system:
Processor: i5-650 (Acer x5900 )
RAM: 4gb (2 pieces of 2gb - 10600)
HDD: 500gb Hitachi
Power Supply: delta 750 watts
Graphics Cards: evga gtx 570

I usually play world of tanks or sniper elite 3.

If you have this kind of system, which would you upgrade first and with what specifications? I might have around 400 - 500 USD budget for this holiday season. Is it advisable to change processor or (motherboard and processor and retain everything)? Or change others then retain the processor/motherboard? I haven't touched anything on the motherboard since buying the system in March 2010, only changing the power supply, HDD, and graphics card.

Any feedback is highly appreciated.

Thank you!

 

[10030026]

Sorry I forgot to include that my system is always turned on. I attached an external drive to my cpu that takes in jpeg and avi footage of my home for 1 ip camera. I am running an FTP server using filezilla.

 

[10030026]

In addition to above, is an i5-760 compatible with my motherboard? Has someone tried that setup?