well that depends on how much money you want to spend.
Assuming you get a 9800PRO, you will notice a phenomenal difference in performance, and you won't realise you are missing out on the benefits of AGP8x because you don't have it. Its kind of hard to explain.
Like if you don't know what your missing, it doesn't matter. (thats my attempt at a dodgy analogy but nevermind :P)
You can always get a mobo that has AGP8x support at a later date, and simply wack the card in and it will be at its true power.