Yes, 802.11ac ONLY works on the 5 GHz band, but:
802.11ac is backwards compatible with 802.11n and other "legacy" standards
802.11ac falls back to 802.11n to serve devices which don’t support it. This works very similarly to how 802.11n falls back to serve 802.11b/g and 802.11a clients nowadays.
802.11ac is therefore backwards compatible with 802.11b, g and n. This means you can buy an 802.11ac-equipped device and it will work just fine with your existing router.
Backwards compatibility is a key point since nearly all deployments will be dual-band and therefore will need to support 802.11n for years to come.
EDIT:
Some devices may only be capable of 5GHz, which means they’d only be able to revert to 802.11n. So if you want to maximize compatibility with older standards, be sure the Wireless Card of your new PC is dual-band.