***********.com provides in their VPS plans:Guaranteed RAM and Burstable RAM. What does it mean? How much RAM shall I get for sure?

Depending on the virtualization software used, the host can isolate certain resources per VPS. In general 'guaranteed RAM' would be the RAM that would always be available in contrast to 'burstable RAM' which may only be available if it's not in use by another VPS at that instant.

For example, if the host (the physical server) has 1 Gb of ram split among two VPS's you would only have the 512Mb of RAM that's allocated for your VPS. However, if needed and the other VPS is idled for a moment the other 512 Mb might be available for a very short time.

Further clarification on that: The burstable RAM described by Brianf only functions that way on a Linux-based server. Windows based VPS servers most likely do not have burstable ram.

Always look at dedicated/guaranteed ram, because burstable ram is only usable while other VPS aren't using the full ram they need. Make sure you have enough guaranteed ram to run whatever you need happily

At least ***********.com states sincerily about guaranteed RAM and burstable as it's important to know the amout of RAM you are allowed to use.

Burstable RAM is meant to be used for short periods of time. For example when you install software, you might need more ram. But once the install process is finished, you don't need it anymore.

Guaranteed RAM is what you need to cover apps that are running all the time, such as Apache or MySQL etc.

The majority of VPS providers state the guaranteed and burstable RAM for their plans. It's as important as storage and bandwidth when deciding on plans.

The only way to determine how much guaranteed and burstable RAM you have for your VPS is usually in the VPS control panel.

All is stated clearly on the web site of ***********.com. Guaranteed RAM is the amount you'll get for sure, Burstable RAM is what you might get but not always. It depends on your web site.

The amount of RAM you intend to go for depends on a few things, first of all the control panel you are going to use (seems like ***********.com offer Cpanel on their account) which means you should go for a plan with 384MB of RAM, also can you be more specific on the type of content you are going to have on your account.

Does the amount of burstable RAM depend on the virtualizatiom technique? So, what's the best option then Xen, OpenVZ or Virtuozzo?
***********.com offers Cpanel plus Virtuozzo, may it be so that chhosing a host with Xen you'll get more of burstable RAM.

No (exception: Windows which doesn't have burstable RAM as was mentioned in an earlier post.). However, the virtualization software does determine how 'isolated' the VPS is from others contained on the host machine. Of course, all claim that one is better due to various specifications.

No. It's based on the amount of RAM on the host machine. Virtuozzo is the control panel used for managing VPS's created by the commercial version of OpenVZ, so it's not actually virtualization software. Also, if I remember correctly it is not compatible with XEN.

Note: cPanel uses a significant amount of resources, so having it on a VPS with less than ~512Mb of 'guaranteed' RAM tends to slow the VPS. A better option might be DirectAdmin.

Not only ***********.com but a lot of other companies provide Virtuozzo on their VPS plans as it is the most usable application for a VPS usage IMO.