To install the Guest Additions (which downloads automatically with VirtualBox), you need to be running the VM and choose Devices --> Install Guest Additions from the VirtualBox main menu.
I found Windows 7 to run quite well as a VM under both MEPIS Linux and Windows XP SP3. As far as I'm concerned, the Windows 7 beta actually runs better than Vista SP1.
You should be aware, though, of some fundamental differences between running an operating system in a VM and running it natively. The most common problem is the inability to use the higher performance features of graphics, audio or network cards. That's because, like most virtualization software, VirtualBox provides the guest operating system with a virtual VESA-compatible graphics card, a virtual Intel ICH AC '97 audio and several virtual network cards. In short, your virtual Windows 7 can't use your high-end graphics card or what-have-you because only the host system has access to it, not the guest operating system, which must use virtual drivers instead.
Still, unless you want to run a high-end game, you're not going to notice these lacks. VirtualBox gives you everything most of you will need to decide for yourself whether Windows 7 will be worth your time. As far as I'm concerned, Windows 7 and VirtualBox are a great combination.
The new, new VirtualBox
I've been a VirtualBox user for some time, and I really like it a lot. It's simpler to use than Xen; it's largely open source, unlike VMware; and it runs on pretty much any desktop operating system you care to name, which is certainly not the case with Parallels or Microsoft's Hyper-V.
In addition, Sun has been working hard on improving VirtualBox. In the last two months alone, the program has received a pair of significant upgrades.
Back in September, VirtualBox 2.0 added 64-bit support. Only a few months later, in December, VirtualBox 2.1.0 picked up better 64-bit support and Mac hardware virtualization, 3-D graphics acceleration and support for VMware VMDF (Virtual Machine Disk Format) and Microsoft VHD (Virtual Hard Drive).
What this means is that if you already have VMware or Virtual PC VM images of your data, you can run these operating systems under VirtualBox. I was able to use a VMDF Server 2003 image and a VHD copy of Vista on both my Windows and Linux systems. If you already have a lot invested in other virtualization systems, this is a darn handy feature.
Then Sun released Version 2.1.2 in January. While it included numerous bug fixes and performance enhancements, the most noteworthy change is that VirtualBox now supports the Windows 7 beta natively. Before this, in 2.1.0, you had to do some tweaking with the audio and networking drivers to get Windows 7 running properly. In addition, if you wanted to use the Guest Additions, you had to run the Additions installation program in XP compatibility mode.
There may be better desktop virtualization programs available than VirtualBox, but I haven't found one yet. Its combination of flexibility, speed and features makes it my PC virtualization program of choice. If you give it a try, I think you'll find you agree.