In most cases, if you already have Garmin equipment, the easiest route to comply with ADS-B is to stay with Garmin. Based on what you gave me below, you would need to upgrade your GNS530 to the 530W (WAAS) and replace your GTX327 with the GTX330ES (Extended Squitter). Most shops will give you a trade-in value for the GTX327 and there are some used GTX330ES units out there on the market. This route will make you compliant with the 1090ES part of ADS-B compliance.
Otherwise, if you choose to keep the GTX327 and go with the GDL88 or GDL84 UAT (978 compliance), you will still have to get your GNS530 upgraded to WAAS and then handle some additional considerations.