I would wait about a month, as we'll likely find out when Leopard is releasing by then. Then, if you decide you can wait for the Leopard release, I'd do that. As then, if there is a small update, it'll likely be around Leopard's release as they'll be loading a new OS anyways. If not, then atleast you get Leopard for free. If in a month we find out Leopard is still 6+ months away, you might as well buy it now.
I, personally, am waiting for the start of summer for the iPod promotion. Hopefully by then, Leopard will be preloaded, they'll be a speed bump, and I'll get both a free iPod and printer. But if I wait that long, I may as well wait for the Conference in August... 😛