Depends on what you need, if it's just a motherboard, new RAM and a processor, you can get top notch material for about 400 EUR.

GA-MA790FXT-UD5P, the AMD Phenom II X4 955 and 4GB of DDR3 ram is a very good combination.

If you want an entire case: pick WD HDDs, and find a case (Antec or something) with a decent PSU (prefer a rather strong one )
Also, I would buy a floppy drive, even though it's nearly obsolete, but it comes in handy at time (for example when you want to install XP and you need SATA drivers). It only costs about 5 EUR for a floppy drive anyway..

read alot, look alot of becnhmarks, find out what you want, and then you will be able to find the best deal whether its a pre-built pc or one that you build yourself.

as for the motherboards; the upgrade ability is really gonna be dependant on the sockets. the so either get an intel with a i7 or an amd with the am3 socket. probably the am3 socket would be a better choice, i think Amd is more determined to make things backwards compatible.  also, if you want upgradability, building your own will almost definately be the best choice, most companies that sell their own pc's use cheap psu's and have weird mounts that make upgrading the psu near impossible.

and while on the topic of psu's not every antec is a good psu. there are several really good psu brands; i think the one that provides the best performance for the most reasonable price is Antec's "earthwatts". and when it comes to psu's, brand is everything; a 500W psu from a good brand can smoke 1000W psu's from bad brands. i think Antec has a 650W earthwatts psu, for around $80, you could probably get it on sale for $70 more maybe even less that would provide upgrade ability, but also fairly priced; the only problem it would have would be if you try to run multiple videocards. a 500W earthwatts psu is enough for any pc with any single videocard on the market right now; so 650W earthwatts would provide plenty of headroom for upgrades. and multiple videocards is very rarely a good choice for upgrades, so dont have to worry about the psu not being substantial enough for multiple videocards.

the most important thing for upgrade abillity of a computer is not the motherboard, its the psu. without a strong psu, not only will you not have the power for new components, a bad psu is probably the most common hardware failure for a computer, and a bad psu not only keeps the computer from working until its replaced, it can damage and kill other pc componants such as the processor, motherboard, videocard; or even start a fire and burn down your house.

ah, misread your post the same thing is true about OCZ and corsair, its been awhile since i did the research forgot how to distinguish the quality corsairs and OCZ's from the mediocre ones.

"hehe I can see you are passionate about fire safety"

actually no, just emphisizing the improtance of a quality psu.

"PS I've been checking this site http://www.cpubenchmark.net/index.php for benchmarks on the different processors, is the AMD AM3 not on there yet or am I not recognizing it?"

the am3 is a socket, not a processor, you would get like an phenom ii x4 if you get an amd. also, that becnhmarking site doesnt seem like the best. basically different types of programs take advantage of different types of hardware better then others. like the i7 is a huge step forward in processing, except for games. Intel wasnt concerned about making it perform well in games when they designed it; so if you look at some bechmarks that dont involve the fps (frames per second)  of games, the i7's will probably blow away the Phenom ii's. but Phenom ii's were designed with gaming in mind, and when you actually look at benchmarks for games, its quite close. individual games might typically perform with a certain brand cpu or videocard, and sometimes the tester is biased or he just accidentally messes up, or a piece of one of the rigs is malfuctioning, so you want to look at the largest variety of sites with the largest variety of games as benchmarks if you're concerned with pc game performance.

http://www.tomshardware.com is one reputable site, but dont go by it alone, look at multiple benchmarks with games.

the case is kinda important but more expensive isnt better always. basically, the case is gonna have an impact on how noisy the computer is, how heavy it is, how cool the parts are, how many drives can fit in the case and how much dust gets on the parts.

you know what blue ray is, right? its basically the new dvd; right now its only really used for high def movies; if you dont plan on watching blue ray movies i wouldnt buy it right now, its really expensive in the future it will go down in price and you could buy it later and add it to your computer. just go to new egg look at the cheapeast dvd burners, get the cheapest, highly rated dvd burner.

if you do not plan on overclocking your cpu, the coolers that give you with your cpu should be more then enough. just dont buy the cpu oem, oem cpu's dont come witht he stock cooler.

another thing to look at different videocards have different PCIe sockets. some have 2x3 some 2x4. some have multiple some only have 1. make sure your powersupply has the connectors your videocard needs. also make sure your dvd drive and hard drive has the connectors your motherboard is compatible with.