I cannot really offer you advise on what components to use, but i can suggest click here I have purchased a computer from this company and was really impressed with it. The system itself and the service i recieved was so much better than other comapnies i have purchased from.
The main reason i bought from them was because they will build a system to your specification. Like you i also wanted the option to upgrade the system at a later date should i need to. The great thing about this company is that they will allow you to do this without invalidating the warranty. Even if you purchase the upgrades from another company. They dont seal the cases so you are free to replace and upgrade any of the components. I have never know a company to do this before but think it is a great idea.
Overall i am extremely impressed with both the company and the system, have a look at their website theres some great systems all reasonably priced, but id give them a call to for advice, their sales department an really good, not like typical pushy sales guys.
Hope you get the system you want!!!