personally, i usually end up upgrading cpu and mobo at the same time, and reuse everything else. ddr2 ram has been around for long enough now, and i've been using the same 2GB ram kit for a few years. works well enough for me. PSUs typically last awhile, since the pin layouts for motherboards is more standardized now. the success of pci-e means you can get any modern video card you like. usually, if the rest of your system is older, the video card is more likely to be bottlenecked by the cpu, but it makes a good stopgap upgrade for the rest of the system, as that same video card will be usable in a new motherboard. basically, these days it seems that any part you're looking to replace, there will likely be a modern solution for you (provided your system came out relatively recently). in the case of cpu's, even if your motherboard can't support a modern cpu, the low cost of high performance, multi-core cpu's these days means you can afford a new cpu and motherboard for a comparable amount of money to a new cpu a few years ago. i remember when dual core cpu's first came out and were several hundred dollars for the cheap ones. nowadays you can get an athlon ii x4 and motherboard for under $200 easily. of course, the higher end stuff is always going to be more expensive, but especially when you look at amd's lineup, it's ridiculous how cheap good processors are. i remember when you'd have to drop a couple hundred dollars for an extra 200MHz cpu speed. nowadays it's $10~$20 for an extra core and $5 for 100MHz.