1) NEVER put your e-mail address on a public forum, EVER!
2) Yes, you can mix and match ram sizes on most motherboards, but know that they will both run as fast as the slowest stick installed.
3) For best performance you want to use the same sized modules in order to take advantage of Duel DDR, which adds ~10-20% performance to your system. Different motherboards handle duel DDR differently; in the case of having 3GB (1GB +2GB modules) some motherboards will still get duel DDR speed for the 1st GB, while others will turn it off entirely. For most boards there is simply no way to know how the system will react, so it is generally best to install RAM in pairs. All duel core Intel motherboards should support duel DDR, and most AMD boards will (with the exception of low end first gen Athalon x2 capable boards).
4) If you are on a 32bit system (windows XP or before, or Vista/7 32bit) then your system can only address ~3.2GB of total ram. To run 4GB or more of ram you need to be running a 64bit operating system (Windows Vista, 7, or the extremely rare XP x64 Professional). Not that this really applies to you but XP64, and Vista/7 Home editions can only address 16GB of ram, while Vista/7 Professional and Ultimate can address 128GB of ram, and server editions can support much more.
5) Use msconfig, and programs like CCleaner to limit startup programs, and then use a lightweight antivirus/antimalware program like Microsoft Security Essentials to keep your system clean. This can make a huge difference in older systems and can be the difference between a usable system, and a system that is painfully slow to the point of being unusable. Defragmenting your HDD can help a bit too.
One last note: Virtual memory is what generally causes system performance to lag, and this is using the hard drive as system memory which is generally very slow. On windows 98/2K/ME this virtual memory was not generally used if you had 1GB of ram or more (though it would use it if you ran out of system memory). For XP and the 32bit editions of Vista and 7 the magic number is 2GB of system memory. For 64bit operating systems the magic amount is 4GB. If this is a home system and you are not loaded down with startup items, viruses, or other malware, then this is a good guideline of how much ram you should have in your system. If you are a power user then simply double the suggestion. So long as you have more system ram available than you are using then your computer should run fairly smoothly.
Best of Luck,