Hi mate
yes,that will be fine .... even though the core 2 duo,s are now classed as legacy products they should be more than capable of running GTA4 at 30fps .... however it also depends on what core 2 duo you have ... for example the E8400 and E8500 sku,s are some of the best c2d,s and combined with the HD6570 should be able to run GTA4 on medium detail and acheive 30+fps ( with games for windows update applied )
however if its a E6400 or E6600 then you may still be able to run GTA4 at medium but the fps will drop to around 25 frames per second .. this is still playable but you should be able to hit 30fps by disabling v-sync and lowering the draw distance ...
that said you can get " core 2 quads" for around £30 these days .. for example the "Q6600" can be snapped up for just £22 from
https://uk.webuy.com/product.php?sku=SCPUBDNA10#.U91YRPldXHQ and this can still run many of the latest games and can be overclocked to 3.2ghz with some simple changes in the bios
however if you do buy the Q6600 or any other core 2 quad check that your motherboard supports it ... even though the socket designation may be the same does not mean it will support all lga775 processors ... its the chipset thats the important factor ...
hope this helps mate ....
good luck !
t