I collect Nintendo NES mainly and am well on my way to a complete US set. The problem is I get sidetracked, and currently I just can't resist the temptation to buy Sega Genesis. I don't know much about the system though, so I was doing a little research. My question is, I found this Sega CD at a garage sale but can't find out much about it. I couldn't find any listed by themselves on ebay, but I'm not sure I'm looking right. Here is the only one I found

http://cgi.ebay.com/ws/eBayISAPI.dll...DME:L:RTQ:US:1

See it's different then all the other Sega CDs I found because the it is for the first generation system as opposed to the second gen where it sits side by side. So what exactly is this called? Sega CD Generation 1? Are they rarer than the second generation? How much do they usually sell for with a Genesis on ebay? Thanks in advance, any and all info is appreciated.