We upgraded the PA28R201 in two stages: first, we replaced one old KX170B with a KX155, which is a very nice, modern flip-flop NAV/COM. It also drives the DME, and includes ILS. That alone was a big improvement.
Then we replaced the other KX170 with a GNS430, and demoted the KX155 to become box 2.
The GNS 430 is very clever: it needs a fair bit of studying the manuals, because much of what it does is NOT intuitive. Above all, you need to program in your route before takeoff. You absolutely still need your paper charts, approach plates, etc - while the GNS430 includes the SIDs and STARS, it's not "approved" for flying them, and anyway includes no vertical guidance.
If you've got a PPL in the right hand seat, and if she's well versed in operating the 430, then it's wonderful
Flying single-pilot, you'd better have programmed in everything you want, before takeoff.
Would I give up the 430 and go back to two NAV/COMs with no GPS? Never! There are some excellent little wrinkles that make life very much easier: for example, if it's a bit murky, you can get lined up on final by pressing GOTO ENTER ENTER very few seconds until the Track Req is the same as the runway orientation. If you have an engine failure, GOTO NEAREST will give you some ideal suggestions and their frequencies.
Other useful bits: a decent intercom, with separate inputs for each microphone (so incompatible headsets aren't a problem); an avionics master switch (saves turning off ten or so little switches); an avionics cooling fan - stops things fading out on a long trip during hot weather.