Answer: With the technology market changing almost daily this fluctuates somewhat, but for £600-800 you can expect to get a mid-ranged PC capable of playing some of the newest games, maybe not at the highest settings but they should be playable.
Information: There's many reasons a system may be running slower than expected, the most common reason is that the system is running low on disk space or memory, this is a surprisingly common issue with pre-built systems.
Answer: The good news is most of the time a simple cost-effective upgrade can usually speed up almost any system. In some cases, some general housekeeping is needed to get a system running at full speed once again.