How do I make the S70 faster and more powerful?

Find a Volvo forum and see what other owners have done. I would start off with a ECU tune, custom intake, headers and free flowing exhaust.

You can install an air intake and exhaust system - http://www.carid.com/volvo-s70-accessories/ Together they will add about 25 hp.

Don't do anything until you've completed Stage 0. http://www.matthewsvolvosite.com/ what-is-stage-0.html If it's got some miles on it (a '98 presumably does), then doing this basic work is absolutely essential before any mods come into play. The first thing that more power will do is show you the weakest point on your car - by breaking that point. Stage 0 will reduce that risk. After that, take a look at chip upgrades, and exhaust. Most cold air intakes don't do a whole lot for 850s/S70s... http://www.matthewsvolvosite.com/ chips.html IPD is my preferred choice, though it will cost more. Unless you plan on upgrading the internals of the engine, play it conservative. Excellent gains can be had with an ecu flash and upgraded exhaust. It will translate to noticeable street and highway performance. Just don't neglect the basics, first.