The Shuttle SH87R6 is a great foundation for a small yet powerful system using the latest in hardware. You can combine a Core i7-4770(K), a couple of SSDs and even a mid-range dual-slot graphics card, for example. It's perfect for bringing along to LAN parties, or you can make yourself a powerful yet tiny, portable PC.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips