Garmin 500 is not worth $350. 500 and 800 are being phased out in favor of new 510 and 810 (which are basically the same except that 510 has a bigger screen than 500, and both have bluetooth connectivity.) Older models can be had for a substantial discount off MSRP.

I already have a HR sensor and a cadence sensor, and I just got a new 800, the GPS unit only, for $325. 500 is OK too, but 800 has mapping (which I do occasionally use) and a much bigger screen. As far as I know, 500 can display 8 data fields, 800 supports up to 10.