Ok, first of all "the card's write speed is of utomst importance." Ok, sure, maybe you should make sure it meets the cams max data rate, but a faster card WILL NOT do anything if your cams data rate doesn't support it.
In this case, the go pro HD is up to 15 Mbit/sec which is only 1.83 MBytes/sec. Therefore even a lolely class 2 (2 MBytes/sec) will work just fine with the GoPro's top data rate.
Anything more then this is basically throwing away money.
And no, the best card for your buck is basically the cheapest that has your required data rate. Most SD cards operating in theses data rates are more or less the same. The memory modules are all from 2 or 3 companies so you should be fine either way. In some higher rated speeds (class 6) there is some potential to get better performance from a high end, name brand card (high vibration situations) but these performance increases are negligible and the value per $ is not good.
Therefore, your best bet: Buy a 32GB on eBay for $20usd shipped. Thats what i', using with my go pro and it's great.
heres a link to the one I bought (note, it's actually a class 6!)