Winegard is good about offering gain and beam-width numbers. The single gain value offered by other manufacturers is ambiguous... it may be the peak gain, the average gain... but I doubt it's ever the minimum gain. (Expect the gain of any antenna to change with frequency. For example, most UHF antennas have better performance on the higher channels.)
If you have a weak signal and know the antenna gain at that frequency, you can use the simple math at the following link to estimate the results.
http://forum.tvfool.com/showthread.php?t=109 (Thanks to @gonzo & @mtownsend for starting the thread.)
Re. PC-card and network attached tuners, my personal experience with the SiliconDust HDHR tuners has been that they work well, but the tuners in the newest TV's are slightly better at holding a lock on a marginal signal. I won't give up on my HDHR tuners though, I have yet to find a better option for recording OTA to disk.
http://forum.tvfool.com/showthread.php?t=820
If your tuner card has a built in 4-way splitter, expect the same effect as an external 4-way splitter (7 to 8 dB insertion loss).