I am looking for a VPS and have found an interesting offer from HostV.com. Everything's o'k with them but I haven't got one thing - their Double RAM - what is it used for?

Are you asking the use of RAM? For me it looks like an attempt to promote the host mentioned. Nice tactic, is it? Next open a thread asking, oh.. they are offering $5 less, what is $ used for ?

I think they double RAM by using additional slot, do they offer double bandwidth as well?
Double RAM will let you say use Cpanel on your account or whatever resource-intensive you wanted but couldn't.
It's also essential to see whether it's double ram for life or not, so ask HostV.com about it.

There are plenty of vps deals now with discounts and double ram, make sure you'll get the support you need and the type of the panel you need.
HostV.com is a project by Cirtex that specializes in vps, so I think their services can be quite reliable.

Double RAM is x2 of the original ram offering.

They offer 128ram. With the current offer, you will get 256ram. They are giving twice amount of memory to your package. More ram means your application can run faster. The more the better.

Double ram just means that they are offering two time the original amount of the package. It is ideal that you get at least 256 ram so that you can run cpanel, whm and other resource intensive things

HostV.com offers double disk space too. This company has many technology partners: Virtuozzo, SWSoft, FFMpeg and others.
Is CirtexHosting.com a part of HostV?

HostV is a part of Cirtex. You an read it on their site.

While it is common to see such partners on a host's site, IMO, just because a company uses certain products, doesn't make them a "partner". In most cases they're merely a customer.

doublem RAM usually make your vps run faster

Double ram should be for life.
Noone would check vps and reduce ram 1 month or 1 year later.