I'm not aware of any site that breaks down features by year.
That would be handy. This site does offer some data:

http://www.volvotechinfo.com/

Other then that, you may just have to use google to search on
things like "volvoe engine designation" to find sites that list
the details. I do see lots of test drives on 98 models on the
net since there was a lot of interest when the 70 series came out.

Big change for 97 was the light pressure turbo in the GLT models.
A 97 GLT wagon would be a nice car. Other markets got AWD on the
97s, too (U.S. did not).

The 70 series, introduced in 98, is basically a "mid-life" kicker
for the 800 series. Safety was improved a bit with improved knee
bolster and slightly stronger B pillers (this gives the side impact
air bags a bit more time to do their job). Side impact air bags
offered head protection starting in 99 model -- earlier side impact
offer chest protection. The 70 series steering column was also
improved to reduce impact forces on the chest (it collapses easier,
I assume).

The 70 series high pressure turbos also had a bit more power (236 HP
and 241/lb ft). There were no 70 series R models imported to the
U.S. market other then the V70 AWD R. 98s may also be found
with manual transmissions.

R models hade larger tires and wheels, stiffer suspension and the
ECU allowed a bit more boost for more power (for a short duration,
something like 10 seconds -- which will get you to 80+MPH). The
70 series ECU basically have the same power limit as R models but
hold boost as long as engine is not over-heating (so slighlty more
sophisticated control mechanisim). The AWD R actually got a bit
more power then the others.

The built-in child seats are optional (my wife's 95 GLT wagon has one,
my 98 S70 T5M does not). I don't really care for it.

The sportwagon was a marketing term for the 850 high pressure
turbo models.