RE: colour temperature - it's your preference. For me, the "ideal" is the 4000K ones, but they are getting hard to find. I think 3000K and lower are too "creamy" for my taste, and 5000K are just a bit too blueish, but not by much. So If I can't get 4000K I'll go 5000. Subjectively, some people find that 4000K or 5000K light seems "brighter" for the same Watts or Lumen ratings.
Re: power, brightness, temperatures. I agree with Micrornd - forget Watts, look at Lumens output. For your situation looking for a significant increase in brightness, try for more than double what you have been using. Operating Temperature (not normally spec'd and NOT the "colour temp") considerations are different for LED lamps. Each such bulb has a "driver" circuit in its base that converts household power to what the LED's inside need. That circuit produces waste heat, but it is concentrated in the base, not in the upper part with a filament like on old bulbs. Light fixture Wattage ratings are based mainly on ability to get rid of heat at the fixture lamp base when using older filament bulbs (depends on base material and wire insulation), and those have the main heat source far out from the base. In your case, the sockets can withstand the heat from a 40 W filament bulb, so it PROBABLY can stand the heat from the base of a LED bulb rated in the 20 to 30 W area. Such a bulb can give you a LOT of light in LED type. For example, the Lohas bulbs linked by Micrornd above use 23 W and claim to generate 2500 Lumens at a colour temp of 5000K. I'm using for high-brightness locations these similar LED's from Energetic Lighting
rated to consume 21 W for 2600 Lumens at 5000K. I also use these for a bit less light from Phillips
using 14 W for 1500 Lumens at 5000K.
Re: Costs and Lifetime, I think the estimated lifetimes quoted by LED makers are unreliable, but do offer some useful info. Forget "years of use" numbers - use only the HOURS specification. 10,000 hours is poor. 30,000 hours and sometime more is good IF you do get that. But it will be at least a couple of years before you know! My inclination is to rely more on well-known makers. I DID keep track on some 60 W equivalent LED's from Phillips I use in a Living Room fixture. They certainly exceeded the spec's Phillips quoted on them - I calculated over 30,000 hours on several when I replaced them, so I bought a bunch more of the same ones. I do notice that many cheaper LED lamps spec their lifetime lower that better lamps.
In your case, OP, you do not need ones that claim to be good for use with dimmers. SOMETIMES you get ones rated that way for almost the same price but probably rated for longer lifetimes. I suspect that these ones have tougher components in the drive circuits in their base to deal with a different type of power input. In fact, I also suspect that what limits the lifetime of LED lamps is that driver circuit. That's not like the older lamps whose lifetime was determined by the filament.