It is not the type of computer (i.e. PC, Mac, etc) that matters, nor the brand of router that matters. What matters is the wireless technology type (i.e. 802.11a, 802.11b, 802.11g, 802.11n, etc). Different technologies run at different speeds, for example 802.11b runs at 11mb/sec, 802.11g runs at 54mb/sec, and 802.11n runs about 275mb/sec. I would get a type of router that is the same for all. If your cable modem or DSL modem is located next to the desktop PC, you may not need a wireless card for it, instead you could get a router with a built in ethernet switch, and use an ethernet cable. As to the Macintosh Laptop, you should see which Airport card it has.
If it is the original Airport card (very old models of iBook), it is 802.11b. If it is the original Airport Extreme card, it will be 802.11g. If it is one of the newest models of Macbook or Macbook Pro, it is likely 802.11n. With this information, I would recommend getting a router that is for the technology your computer has. Often the newer ones are backward compatible (except 802.11a), but this sometimes causes them to run slower. For example, a computer with 802.11n card can be used on a 802.11b or 802.11g router, but at a slower speed. Additionally, a 802.11b router can allow 802.11n computers to connect, but at 802.11b speeds.