oblivion yes, crysis no.
to play crysis maxed with AA etc.. you need SLI or crossfire, or a dual gpu card.
it's really quite graphics intensive i suggest getting the HD 4850 though. with a light overclock you can play it on medium settings with good fps. medium settings is still very good for crysis.
personally, i'm probably going to get another HD 4850 and o/c it to 4870 speeds. my card currently has 70% of the 4870's bandwidth and 100% of its processing power. it barely out-performs mine. of course, i did a pencil mod on both gpu and memory with the accelero s1 rev2 cooling it. currently running 35C under full load