IDE is for older legacy drives that use an IDE connection (big-ribbon cable). AHCI (Advanced Host Controller Interface), is basically for drives that uses SATA. Use AHCI to get the performance benefits from an SSD.
In order to answer your questions:
1) Yes, it will load faster. But SSD's do not increase performance such as frames per second.
2) Yes, I agree with most of the points on there. Personally, I turned off pagefile, superfetch, defrag, hibernation, and search indexing.
3) Of course, when you first install Steam you can custom install the program on to your HDD. (whenever you install a new game, all your games will install where you installed Steam).
4) Any motherboards that has SATA ports and AHCI in the options will have the TRIM function.
5) I have a Crucial M4 (SATA III) plugged into a SATA II port. It's fast, boot time and everything else loads faster and it's amazing.
Sorry for long wait on reply. I heard that SSD's can be formatted to IDE or AHCI modes, whats the difference?
Tip: NEVER FORMAT A SSD.
Now, IDE and AHCI are modes that your motherboard/start up screen will detect your drive setup. IDE is for older OS's(Windows XP) but work for all future ones as well. AHCI is for the newer standard, Windows 7 specifically because of TRIM support. IDE or AHCI is a BIOS setting, not a OS one.
Im not actually building the computer myself, im going to www.dinopc.co.uk and ordering from them. I've been told at the end of the customisation and purchasing procedure there is a comments box which will allow me to add notes for who ever is building this computer. When i recieve it, how will i know its in AHCI?
Im only asking because when i got my laptop the 2 HDDs were meant to be at raid 0 and i never noticed they werent for ages.
3. Is it possible to change where my Steam installs to? I'd like it to go to my 2tb HDD.
4. How do i know whether the motherboard im going to get supports TRIM software?
Finally, 5. Is there anyone who uses a SSD for gaming that can give me a realistic life time on the SSD?
1) It will not increase FPS, but if it pulls maps of the HDD, then an SSD will decrease the time it requires for those maps to be pulled by a very large amount.
2) Somewhat, but its not necessary. As long as TRIM is running, you will be fine. No need to bother with any of that, except do not defragment.
3) Yes, steam installs games to its own installation directory. If you want games to be installed on your HDD, install it on that HDD. You cannot install steam on an SSD and then change game installation directories, it will install to is own
4) Its about the SSD and OS. Windows 7 and 8 support TRIM, and so do some versions of Linux I believe. Not sure about mac OS.
5) Easily 5-10 years. SSD's have a limited lifespan, but its far longer than any HDD.