The card's speed doesn't really matter... What matters is: how fast the dsrl can write on it. The card's speed is just a guide number, to know roughly what the card is capable for.
The other thing: Everyone comes with "i never had any data loss or problem with my *insert random company name here*". I have a bad news for you: Almost every card uses the same manufacturer's chip (samsung, hynix, etc...) so the reliability is just matter of luck. (but i don't think that this is a huge problem nowadays)
The most important thing about a card is the control "panel" (basicly one chip), this thing defines the true write speed of the card. Just think about it, every card type (even with the same "official speed") produces different result with the same setup, and this "panel" is the reason for this (but the differences are not too huge) In my experience, but online reviews claim the same: sandisk's are the fastest cards in this matter (for canon dslr, maybe for nikon for example lexars work better, i don't know)
3 years ago when i bought my 40D, i had the same problem, and i saw a test (i don't remember where is saw that) where they showed that extreme IV-s have the same speed in the 40D as the extreme III-s, so in this case 40D was the bottleneck. So you don't have to buy always the fastest card, in some cases its just an overkill. I used several sandisk and kingston card with different size and speed (4-8gb sandisks from extreme II to extreme IV, and 4-8-16gb kingstons with 66x-133x-266x speed) In my experience, sandisk extreme III-s are the best. But I'm sure, that they improved the write speed of the newer dslrs, so maybe extreme III's write speed by now are no longer faster then the dslr's write speed.
In the old days (40D, 5D mark I) dpreview done some test for this (under page: "timings and sizes" or "performance") but they gave up this good behave.