As long as it's a PSU from a trusted brand (Antec, Corsair, XFX and Seasonic are some of the best), then 500w is plenty! The 7790 isn't a very power consuming card, so 500w is definitely enough for that system!
I just find modular PSU's are easier in short. You can select the cables to plug in and the rest can go away until you ever need them again. keeps your system nice and neat lessening the amount of cables in your system. Air flow can be increased because of less cables. And for under £5 more go for it.