If you are already familiar with ESRI, then you should look at the ESRI Flex API ESRI Javascript API. You can use both of them to fire up Server side Geoprocessing services.
There are so many ways to skin the cat...
You may want to check the Publishing a GeoProcessing Service section of ESRI help
The basic idea is this... you use ArcMap/ArcCatalog to author a Geoprocessing Model (I am assuming you are familiar with this), which you later expose in an ArcGIS Server as a Geoprocessing Service. Publishing a model as a service is pretty easy provided you already have an ArcGIS Server configured - see the links I provided you above.
Then you can either use the free ArcGIS Explorer application to consume that GeoProcessing Service or write a webapp using Flex, or Javascript or DotNet or even python to consume that webservice.
And yeah, don't consume the webservice synchronously - stick with async.