Don't reduce case quality. You don't NEED to, and a decent case will last for years, if necessary, without bits dropping off.
The only limits to the capabilities of that build will be graphics card. It can cope with more than a HD6870 can. If you want to add to system, drop in a 128GB OCZ Vertex4 SSD, for OS, and main games. Makes everything feel "snappier".
You wouldn't need higher PSU if you wanted to upgrade GPU. Virtually any new GPU is covered by 550w, and the next generation is likely to have even lower needs. The only two simple ways system could be "improved" would be to use a "K" processor and overclock (adds about £75), or add an SSD, for OS and main games (adds about £85).
The HD6870 WAS a good card, and still is pretty good. I would think your best option is to use that for a year, or so, then get a HD8xxx or GTX7xx.
You could use higher wattage PSU, provided it was as good quality, but just don't think you need to, especially with a locked processor. Short of crossfire, nothing your LIKELY to do will require more than 550w.