Overclocking Hd 7850 XFX Core Edition

Hi, im kind of new at overclocking, so i hope you guys can help me...

Ive just bought an hd 7850 XFX Core Edition (860 Core Clock - 1200 Memory Clock [Stock clocks]) but i overclocked it with MSI After Burner @ ( Core V [Option Disabled] Power Limit [15%] Core Clock [990] Mem Clock [1400] ) and it ran pretty good with the highest temp @ 73° but my questions are:

Whats the difference between Core Voltage and Power Limit? or is it the same? I heard that changing Core Voltage degrades electrical pathways faster... but as this option is disabled, and im not tweaking it... I dont know if power limit affects the same way as Core V.

Ive seen some reviews about some hd 7850´s with 1050 Core 1450 Memory and 20% Power Limit (the max you can Overclock it with AMD Catalyst and MSI) and they say they could push the card even more than this... but id appreciate some advices cuz i dont wanna screw up my gpu...

I was thinking about leavin my card just @Stock basically because my rig is old, and this is the main problem about my low fps...
Ive been playing DayZ, Diablo 3 and Crysis 2... DayZ (drops @ 20fps while im @ cities or in towns, with Diablo III no problems, but with Crysis 2 i just can play @ Very High Spects @ 1920 x1080p, BUT if its safe just as i am overclocking it, i wouldnt mind leave it that way...

BTW my rig is

Cpu:Core 2 Duo e7400 @ 3.5 Ghz (will change it next month for i5 2500k) Ill try to look up for a 3550K
Mobo: Dell Motherboard (will change it next month for some z68 z77 mobo)
Memory: 4Gb Ram DD2 (will buy next month 8gb ddr3)
Video: XFX Hd 7850 Core Ed.
Case: Dell Inspiron 545 (will buy next month Haf 922 Cooler Master)
PSU: Corsair GS600

sorry for my bad English... And Thx in advance...