According to the Sibelius website, the system requirements aren't all that demanding by today's standards. Info can be found here:http://www.sibelius.com/products/sibelius/7/system_requirements.html
That being said, just about any dual core or better rig will do the trick. If I were making this purchase, I would consider one of these:http://www.newegg.com/Product/Product.aspx?Item=N82E16834157426
Very capable machine. 4 cores. 4 GB memory and a large HDD. HP makes pretty good stuff. If this is too expensive, give me a budget and I'll point you to another system.