Depends whether your current RAM can run cas2 at 133MHz - if it can, then you could get another stick of PC133 cas2 and run all your ram at 133, cas2. However, if yours only runs cas3 at 133, there's no point getting PC133 cas2, as you'll have to run it slower. As you have the A7V you won't be ocing the fsb much, so there's probably not much point in PC150 ram - especially if your old ram can't take it. You won't be able to put DDR in the A7V, you need a new mobo.
I would suggest you think about whether it's worth getting more RAM at the moment - it is cheap, but 256 is plenty, and if like me you're planning on upgrading to DDR in the summer then it'd probably just be a waste of money getting SDRAM now. Just save the cash and wait for DDR mobos and DDR RAM to become readily available and bug-free.