Replace 7 speed 14-34 cassette
As long as you're not adding larger cogs than you've got on right now, your existing chain should be a functional length.  

One more thing to consider is that if your current chain has significant wear, running it with a new cassette can damage the cassette (effectively very rapidly turning your "new" cassette into your "old" cassette), so even though your old chain should work fine with your new cassette, it may be worth it to replace it anyway.