The pix 501 is a cisco firewall which is capable of doing site-site VPN tunnels which in your case looks to be very benifical and cost effective. And yes, would enable you network access to the infrastructure in the old building.
If you look at the wireless solution, i would look towards the high gain antennas, such as the AIR-ANT2410Y-R or better yet 13.5 dBi Mast/Wall Mount YAGI. I wont go into a linksys vs cisco debate, but for the business application i would only go with the cisco gear.
This is the small business forum, and the wireless solution requires directional antennas on both buildings and those antennas are not cheap.
If it was my personal network i would weigh both solutions and identify the flaws. If your ISP connection goes down on either end, your vpn solution goes down with it. Wireless requires some mroe time and expertise to ensure a safe and proper install. (lightning and proper alignment between the two sites).