I replaced the 4 year-old OEM in my '98 C70 with the Motorcraft replacement. This is what my Volvo dealer uses if you decide not to go with the Volvo OEM. It is half the price ($55.00 vs $128.00) as the Volvo battery and has more cca than the OEM. And it fits perfectly.
Butchman

DenzO,
Yes, the Optima batteries are good. They are a sealed cylindrical celled battery. They come in two versions, red & yellow tops. One is a deeper cycle battery as I recall, but I don't remember which one is which.

As an alternative for comparison, you might also want to check out Interstate batteries. They too are a very good battery. They use to pull them off the shelf dry and only activate them when purchased, guaranteeing an extremely fresh battery. Don't know if they still do this or not. Last time I purchased one was (4) years ago, and still going strong. Either way, you wouldn't go wrong with either manufacturer. Good-luck with your purchase.
Rich - Ashburn, VA

1. Yes Optima is as good as advertised. Since I'm not selling them I won't get into sales pitch. I just installed one two winters ago and never had any problems starting my car even at -30 degC. It turns engine at this inhuman temperature as fast as at +30 degC.
2. I don't remember what size I installed. The tray was OK. Terminals were not. I ended up upgrading them but it's well worth it.
Happy upgrading.

Why not go with OEM? Competitively priced (at least when I last checked), correct specifications and size.