How much core/engine clock needed?

[MRCmegarallycar]

Hello, im looking into upgrading my GPU from 256mb ddr3 to 1gb ddr5, but the core clock is seems to confuse me. What is the core clock responsible for and how much is needed to run games at medium settings. The main reason for upgrading is soo i can finish the game Sniper Elite V2 at a decent resolution and frame rate. Also what is memory clock responsible for? I imagine it being like ram frequency, such as 1600mhz.

http://www.amazon.com/Sapphire-Profile-PCI-Express-Graphics-11202-10-20G/dp/B0095MRLJ2/ref=wl_it_dp_o_pC_nS_nC?ie=UTF8&colid=2A5DUPMQZIVCW&coliid=IC4SHDSJAT5UK
Im looking into this card to run Sniper Elite V2, its core clock is 800mhz which is the same core as the 256mb ddr3 one i have now. For memory it says 1125mhz (4.5gbps), what does that even mean!? Looking at other cards with 1gb of ddr5 for memory it says 5000mhz, why is this one 1125mhz?

 

[Shadowblade2652]

if you scrolled down it said the memory is clocked at 4500 MHz not 1125MHz