Digital music advice

So I'm new to digital music and doing some research.

I'll be using a SD card in my Treo phone/PDA. I have a 512 MB card so I'm not looking to put my whole CD collection on a portable device, but just some songs to bring and workout to.

My question for you all is---what site(s) do you use to download for non-iPod devices? In looking around, it seems like Wallmart.com of all places is one of the easiest to use. However, I am not a Wallmart fan and if there is an easy to use site, I'm open to others.

I'll get software for the player that will play MP3s and WMAs, though I'm looking a bit more in to WMA format since they compress a bit smaller--comment or thoughts on formats???

Thanks so much