Each architecture is different; it's best to check. Newegg lists it on the stat's page; as does Intel.

It's probably worth comparing more than cores/hyperthreading/clock speed as well; e.g. generational improvements in overall architecture. Though I've never went that far... For myself, I went from Quad-Core (no HT) to a six core (AMD, no HT) to a Six Core (w/ HT); all were clear improvements. Maybe next time I decide to light some cash on fire I will be faced with that decision.

Is hyperthreading an upgrade? Absolutely. will it improve performance in gaming? Usually, yes. Is it necessary to your application? Who knows.

Pinot's advice to the OP was sound. In your case its less clear, but not a bad idea. Though it probably boils down to what you can upgrade without changing mobo. I would pick a hypothetical CPU and Mobo [that is within your near-term budget]; and only upgrade components for my current system that I could transfer to that mobo. If that limits you too severely, it's probably time for a mobo change. If it doesn't; try just adding ram and upgrading your graphics. Depending on how old your mobo is, this might even be a non-issue.