many people have allergies, so carpet should be your solution only if you are going the cheapest possible route. as a home buyer (who doesn't have allergies), hardwood > tile > linoleum > carpet.
countertops: you are going to have to go with real stone (granite, etc) if you expect to sell >$300k. try to pick a color of something that goes with your floor, and that looks 'new.' females pay a lot of attention to the kitchen, so you should consider replacing your kitchen sink if you are replacing your kitchen counter. get a sink that mounts *under* the counter, not overlapping it. vessel sinks are hot items for the bathrooms, save yourself a ton of $ buy purchasing them online (along with the vessel sink faucets) instead of at lowes/home depot. newer cabinetry has hidden hinges, cheap updates include simply adding stain to the cabinets and adding new hardware (drawer pulls and handles). note that they have do it yourself kits at the home improvement stores to stain your current countertops and make them look high end; these kits are expensive, run the risk of not turning out well, and it'd be cheaper and safer to just buy a counter that looks like stone.
If you dont have crown moulding yet, it is one of the best architectural 'wow' factors you can add for the money.
Assess what price you want to sell at, then search for houses that are +$50k that have sold nearby. you want the "look" of the more expensive houses, because your buyers will have looked at those homes and find yours as having similar features buy a 'better bargain."
as far as ROI, it used to be a lot more straightforward... With the market to sell homes so bad, you either need to take the hit and sell it lower than you'd like, or take the hit and put money into it to make it look updated and modern in order to sell it. my guess is that it turns out to be equal in the end on whichever path you choose... For example: While you might put $40k into a house and get a +$25k increase in sales price, if you didn't put the $40k into the house, it would probably take much longer to sell and you'd end up taking a -$15k hit after the buyer's inspector and appraiser and real estate negotiations.
The best ROI that you can get for your house will undoubtedly be your investment in selecting a Realtor. I am likely to get flamed for this, but I wouldnt hold onto a Realtor longer than 3 mos, 6 at the most. and interview at least a half dozen before you make a selection. find out what they will do for you: open houses are a waste of time, open houses where they invite other realtors are helpful. printing out paperwork to leave when the potential buyers come in that outlines what is new, what is improved, what is great about living there is essential. if you arent getting 2-4 showings per week, either your home is priced very wrong or your Realtor is lazy. the best way to pick a realtor? Pretend you are a buyer -- Call and view some homes that are close to your price point and near where you live. See how well you are treated. do they guide you through the home telling you what features are nice/significant/valuable? do they let you wander while they make phone calls in the entryway?