The number of contacts will depend much on the band conditions. But to get a feel for the number of points you might gain for your efforts, you might take a look at what participants in 2012/2013/2014 managed to achieve. Here's one sample: http://k8bxq.org/node/7
To gauge the rate of growth in popularity (to get an idea of the prospects for next year) take a look at the ARRL's FD results, where you'll find totals for digital contacts over the years:
Each contact counts the same as a CW contact (x2) but you don't need someone with CW skills, and digital operating skills are a good bit easier to learn than CW... so a digital station might provide a bonus in terms of available skilled operators.
You'll need to take the same precautions to avoid receiver desense and related strong-signal issues, as with other modes. But many digital modes can perform effectively at low power, so they can be less prone to inter-station interference on FD. Additionally, many of the digital modes are more immune to interference, and are less likely to be troubled by the CW ops. With care, and adequate antenna separation, you might find that the CW and digi-station can share the same band without serious issues - which can be a real bonus when only one band seems to be open on FD.
PSK31 has been around for a very long time, works great with low power and high noise, is very straightforward to use, works great on HF, and should definitely be one of the modes you'll want to use. But if you can be flexible, you'll find that changing to another mode after exhausting all the PSK31 ops on a particular band can help boost your numbers. With a computer running a flexible digi-mode app you will have your choice of virtually any of the HF digi-modes available at your digi station - so plan to use them all!
You might find this blog to be of interest: http://www.w4cn.org/about-ham-radio/digital-modes.
FB OM GL FD