what brands are the best?

If you have enough money, get the "professional" SanDisk Ultra II or Extreme III thingies.
Transcend and Toshiba are good too.

SanDisc has always been my preferred choice of brand.

i got a kingston 'elite pro' 50x 1gb one and its just great.

50x or 150x? PQI has 170x 4GB USB so some are going to release 170x or faster SD and CF cards as well sooner or later.

I also say stick with SanDisk Ultra II or Extreme III SD or CF cards.

Never had one go bad, unlike Ritek and "Adata MyFlash". (stay far far away from those two).

Ritek make SD cards? Oh dear.

CORSAIR 133x 1GB, in my digital camera.

Read speed: 21 MB/s
Write speed: 15,5 MB/s

Lifetime warranty!
Price ~ $21

Yep, and I lost a whole day's worth of photos on a Ritek card.

It's very easy and cheap to "make" flash memory cards or sticks. It costs billions to tens of billions in USD to make the NAND flash chips inside those cards and sticks. Only the very large players like Samsung, Intel, AMD, Toshiba, and several others have the physical capacity to design and produce them.

where might this be?

I got a Sandisk it just works fine.

Scan Disk has never let me down.

Sandisk has never let me down as well, but you have to pay for it. The Extreme IV is probably the best and fasted, I achieve already best possible performance on my Canon Powershot A710 IS with a 2 GB Transcend 150x SD card. For instance I the card is fast enough to take more than two 7 MP pics per second in burst mode and when I stop the buffer of the camera clears immediately. Thus at least on the A710 you cannot get better performance with another card. The Transcend 150x did cost only one half of a similar Sandisk Extreme III.

I've used KingMax for a whole two years on a regular basis - their Platinum Pro edition is very very good - because it's made of ceramic material and it won't break. Similar is the ATP Pro or something which is ceramic package too - their plastic cards are also fine. Stay away from no-brands, and brands like ASTONE, RUNDISK etc. Lexar is also a good one to go for - their warranty is better - similar to KingMAX, although Kingston also gets my recommendation. Sandisk is always more expensive from experience.

Is Kingmax made by Kingston?

Nope. They're two completely different companies.

Kingston is an Australian Memory supplier [brander ... maybe a manufacturer but I don't think the chips are their own] which offers lifetime warranty on most of their products. They're pretty famous for memory and have been around for a long time.

Kingmax is a much "cheaper" Taiwanese memory manufacturer - they make their own boards and chips for the modules and typically offer only a 5 yr warranty. However, they are innovators in the memory area and typically offer products at a much more reasonable price. They produce a range of products [Platinum SD] which has their patented PIP pacakge meaning that the whole SD card is manufactured in a ceramic package rather than the normal plastic case [which I find cracks when you insert/remove the card on a regular basis - and for the protection of investment - I recommend ceramic cards]. These ceramic cards are also water/dirt/dust proof - a reviwer also had an ATP card that was similar using the same technology and chucked it around and dunked it in water overnight and had no problem. I haven't had a problem with kingmax cards and my recommendation comes for their reasonable price and innovative technology. Their Kingdisk line is a cheaper card, without ceramic case. The kingdisks not as fast as most other cards which is sad however, my 5MP canon doesn't write so fast as to make it noticeable even in burst shot mode.

anyway to use sd card for your existing stereo????

If the stereo has SD memory support - you can just plug the card in and it's all systems go.

If it has USB stick support, you can grab yourself a cheapy USB to SD reader [just one slot though, some other multi-format ones don't seem to work] and put the SD card into that and it'll work as well provided there's enough power from the stereo.