a 5670 wont let you play on ULTRA tho either.
When they say it will "Play" the game means only just.
like saying a 1.3ltr 4cyl "Can" pull a boat. definatly would not want a big boat or live in a hilly area.
we need to know what power supply you have.
for high / ultra you would need something like a 6950 but that will need a 600watt or better power supply
$250ish for the 6950
$100ish for the PSU