If you're going to be sat for long periods of time, whether gaming or working, a good chair is essential IMO. They are not cheap, but they are good value for money in that they tend to stay in good condition for a much longer time and don't slowly cripple you. My Herman miller mira is about 6 or 7 years old now, it cost £550 new (no arms). Before that I had three cheap office chairs at ~£150 each non of which lasted for much more than 2 years.
It amazes me that people will put down £800 on a top end GPU and won't spend half of that on a decent chair..