Top of line, should start around $1,000-$1,500 exl. monitor. You have a decent mid-range then. Should play all modern games. If you really want the top: $2,000 and more.

My personal strategy is settling for the (higher) mid-range. The top is often way too expensive for the little extra gain.

When you plan to buy a new PC for M2TW: do it only just a month or so before it ships. You'll always get faster hardware for less money when you buy later. Normally :)