I'm thinking you're going to be pushing it on that psu and I don't think the cost of upgrading the psu is going to be worth the performance boost. The only way it would be worth buying a new psu would be if you plan on buying another GTX 460 soon.
Are you upgrading your mobo just for this? If you will probably be dissapointed because you'll see a minor performance boost in the few games that use physx. The majority of games you'll see no boost at all.