You are very safe if you go with either the Pentium 4 or AMD based machine. The Celeron is fine as well and is cheaper but is pretty scaled down in terms of performance.

you should be able to get a machine with the specs you need for about $800. You can even get them at Walmart (http://www.walmart.com/catalog/product.gsp?product_id=2134421&cat=3951&type=19&dept=3944&path=0%3A3944%3A3951)