I'm sure this topic is covered in the archives of this forum, but I'll chime in anyway. Firstly, this artice by John Klett is quite useful to get you started:
http://www.technicalaudio.com/readin...1081/1081.html
To paraphrase him, he recommends replacing like with like - axial caps with axial, and radial caps with radial.
In terms of voltage ratings, it's okay to go upto the next voltage rating - so 6.3V for 4V and 35V for 24V for example. It is also okay to uprate the capacitance value, so just use the nearest value above the specified one - never use less.
Always use 105 degree C rated caps - these Neves get pretty hot!
Hope that helps.