It's All in the Details
Try to think of creative ways you can make a big impact for less money, such as installing an outlet in the living room floor to avoid sloppy lamp wires or putting recessed lights over the master bed with individual switches above each bedside table. I love to allocate space in a front hall closet or obscure area for media recharging -- a center with lots of plugs to store and charge digital cameras, video cameras, iPods and cell phones helps with organization and cleaning up clutter.
If you replace interior doors thinking it's a simple high-impact, low-cost item, take it one step further and consider you probably need hardware and trim and should allocate funds in your budget for a carpenter to install them.
By the time you pay for everything associated with one little door swap, what is the total cost and is it worth it? Same with cabinets, windows and just about everything else. Don't forget the "extras," which can add up quickly!