The key is understanding the difference between level and volume. Level is an objective measurement, but volume is subjective. Perceived volume is a function of average levels rather than peaks. "Loud" mixes have a low peak-to-average ratio, also known as "crest factor". Lowering the crest factor (decreasing the ratio) raises the perceived volume, even if peak levels remain the same.
Having too high a crest factor (high average-to-peak ratio) isn't good, but having too low a crest factor is worse. Mastering engineers get paid for what they do because they've learned how to strike the best balance for a given genre and target distribution medium. Fortunately, meters exist that will show you perceived volume in LUFS. Pick a target and adjust your average level to hit that target, and you're good to go.
Your best friend is the mastering limiter. Upgrade to SONAR Platinum and you'll not only get a good one in the bargain but a LUFS meter thrown in as well (the Adaptive Limiter mentioned above). In the meantime, there are free or inexpensive limiters and meters that will fit the bill.
For free, grab
Limiter No. 6. It's not the easiest limiter to use, but once you figure it out it's quite good. Another one that's also free but easier to use is Meldaproduction's
MLimiter, part of their free bundle that has lots of other goodies too.
Also for free, get a copy of
Voxengo SPAN. It's not a LUFS meter, but it does support the K-14 scale, which is close enough. Choose "K14" as the meter and adjust the limiter until your song hovers around the 0 dB mark, going up to about +4 dB on the loudest passages. That'll sound pretty fat in your car.
You still might have to turn up a little louder than your commercial CDs, but that's because they are probably overcompressed, which was the norm until very recently. If you don't want your mix to sound dated in 10 years, use the K-14 target and turn the volume up a tad.