This is probably a little different advice seeking than you probably see but please, bare with me.
Approximate Purchase Date: In 2 months
Budget Range: 500-800€
System Usage from Most to Least Important: small to medium server
Parts Not Required: keyboard, mouse, speakers, monitor, CD/DVD-Drive, video card??
Preferred Website(s) for Parts: needs to ship to EU
Parts Preferences: Intel CPU, Asus mobo
1. I plan to use the server mainly to run apache, mysql, some web services, game bots, could be some smaller game server and stuff like that.
2. Mysql will probably be the most demanding service running, so I am thinking it might be a good idea to use an SSD disk. Since I need some redundance too, maybe it's a good idea to have 2 SSDs in RAID (which one?). It is not file server so I do not need a large storage. 250-500GB would be fine. If I needed more storage I wouldn't even think of SSD but this way it's very tempting (also to reduce the noise). Need an advice on that.
3. 8GB of RAM for start, expandable up to 32 if I ever need it. What brand would be the best for durability? Server will be running 24/7.
4. I am thinking of i5 CPU but not sure which one. 3.0+ GHz, tried to compare them on Intel website but the specifications of all the different versions (2550, 2500, 2400, 2450, ...) are almost identical. I need an advice which one to pick because I don't see any difference between them except a few hundreds of MHz which doesn't really mean anything.
5. I'd prefer an ASUS mobo, probably some mid class that should survive for years.
6. Do I even need a video card since I'll be using the monitor just to install the OS and probably never use it again (once SSH is up and running)? Does mobo have integrated graphic that would be enough for this or do I need at least a cheap video card or an Intel graphic that comes with their CPUs?
7. Server might be located in my room so I'd need it to be silent if possible.
Samsung SSD's are fast and reliable in comparison to Sand Force controller based units. So I'd say that's the way to go there if you want reliability. SATA III based SSD's will give a pretty good jump in speed over SATA II based units at the expense of....cash (what else?).
As for a CPU I'd go one of two ways. Get an S series like the Intel i5 2400S which is clocked low and has huge Turbo frequency boost, or the slightly more expensive 2500K which is unlocked and will allow you to set your own stock frequency and turbo boost clock rates in excess of what the stock settings are. For example, you might choose to run at a stock 2.5GHz with four active cores, but Turbo up to 4.2GHz with one or two active cores and thermal headroom available. This is pretty much how the 2400S works, but the 2500K gives the advantage of throwing overclocking in there and being able to control the CPU frequency exactly as you wish for around the same price.
As for a motherboard there are a number of options, here they are in order of increasing cost...
H61/H67 - Home user board. Can us SB/IB integrated video, but no overclocking
P67 - No integrated video, but tons of OC features
Z68 - Integrated video and overclocking, plus SSD caching and switchable graphics.
Z77 - Z68 plus USB 3.0 and PCIe 3.0 and better swichable graphics
I'd go with Z68 if I was you. P67 is end of life, and H61/H67 are pretty crippled from a BIOS standpoint if you want advanced features. Asus makes great boards. A micro-ATX Z68 would fit the bill in your case and runs around $120-150 depending on what features you want (i.e. ports, legacy PCI slots, etc...).
As for RAM just about any name brand manufacturer will do. I've had very good luck with Corsair products, but Micron, Crucial, Samsung, Mushkin, etc... are all very good. Avoid low volt models and stick with DDR3 1333 or 1600MHz rated at 1.50V and a CAS latency no greater than 9 for optimal performance.
No, if you can find an ATX board then go for it, especially if the price isn't much different. The mATX boards are just typically less, and I figured that if you don't need the PCIe slots then why spend the extra money. I don't know which two you're looking at specifically, but as long as the ATX size board has the features you need go for it.
I would also recomend you take a look at MSI boards, they're high quality like Asus and a lot of times they come out on top in terms of idle system power draw. Here's a good write up from Tom's explaining more...
I've actually got that MSI Z68A-GD80 in my gaming machine and love it. I had boot loop problems that turned out to be memory related and MSI exchanged my board via RMA after 6 months of ownership with no questions asked and minimal down time.