I have the same card.
No the mem and GPU should not be the same.
I like to use ATITOOL, for my overclocking, as it has a built in 3d veiw and detects even the most minute artifact(the card messing up). I got mine up to 450/400 when its very cold, but the most stable i got was like 425/380. Like GameGURU said, just take it up a few mhz at a time and test it. Make sure you test it when its all heated up, since thats what its gonna be at unless you game for 5 seconds then turn it off for 5 mintues.
Q6600 G0 @ 3.80ghz
MCR-320-->Dtek Fuzion V2-->Dtek Fuzion GFX2-->Swiftech MCP-655-->
Asus P5K deluxe
4x1gb Crucial Ballistix @ 846mhz 5-5-5-15
3x 250gb HDDs
Coolermaster 690 w/ water cooling mods
3dmark06 - 15,602
SuperPi 1M - 12.421s