Hi Salomie,
I've got c3500xl switch working with pix 515 and also have 3COM 3300 superstack switch working very well with pix 515 as well. Just one thing to remember - make sure the port speed matches correctly with the speed on the ethernet card of the pix.
PS. NetGear is also good too - that's if your customer is considering cost, but I'd recommend the c3500xl switch.
Hope this helps -