The precise amount reserved depends on devices installed. To quote Microsoft, "for example, if you have a video card that has 256 MB of onboard memory, that memory must be mapped within the first 4GB of address space. If 4GB of system memory is already installed, part of that address space must be reserved by the graphics memory mapping."
The manager lies
Typically, with 4GB installed and running Vista 32-bit, Windows will report somewhere between 3GB and 3.5GB of RAM available for use. However, with the release of Vista SP1, Microsoft has tweaked how Vista reports available memory. In System Properties, the full 4GB will be displayed. Fire up Task Manager and select the Performance tab and the truth will be revealed about the amount of memory that is actually available for the operating system and applications.
As for 64-bit versions of Vista, they have their own memory addressing limitations. However, these are not in fact due to any of the mathematical constraints of 64-bit computing, but are actually a function of Microsoft's product differentiation policy. Defective by design? BadVista.org thinks so…
Vista 64-bit Basic tops out at 8GB, Home Premium is good for 16GB and the Business, Enterprise and Ultimate will cope with a giddy-sounding 128GB. All of these assume that the system sports a 64-bit capable CPU, which includes all Intel desktop CPUs since the Prescott revision of the Pentium and AMD processors from the original Athlon 64 of 2003 and beyond. Motherboard support is also an issue beyond 8GB.
Four is more
We're guessing that you've already got a pair of 1GB sticks in your rig, but in order to show you what sort of increase is possible, we're going to use complete kits for the benchmarks. Enter, therefore, our candidates for demonstrating the upside of more memory, namely two 4GB kits as supplied by memory specialist Corsair. Yours for around £75, the 6400C4DHX is a DDR2 kit composed of a matched pair of 2GB 800MHz DIMMs.
Representing the latest DDR3 technology is Corsair's equally tongue-twisting 1600C9DHXNV kit. Once again, this is a pair of 2GB DIMMs, but this time running at 1,600MHz, which is at the top end of the performance stakes. While the £275 typical price point rather undermines the affordable 4GB argument, this kit will do a nice job of adding context in terms of clockspeed.
First of all, adding extra system memory is not a magic bullet that will kill all your performance problems. Many metrics of PC performance – including Windows boot times and most single-application benchmarks – get little or no benefit from extra memory beyond 2GB. If the application fits inside available system memory, adding more will have zero impact. However, bung a spot of application-juggling into the mix, and the resultant performance delta can be enormous.
A lot on
Imagine, therefore, the following multi-tasking scenario. You're happily hacking your way through a large library of high-res snaps from your latest holiday. Perhaps you have Photoshop open to buff up your mediocre photography skills. Maybe you're also a web junkie and as usual have whole heap of web pages on the go, too. OK, that's slightly contrived. But you get the idea: you're deep in multi-tasking territory.
If you're a gaming freak, eventually you'll want to fire up your favourite frag-fest in search of light relief. You then have the choice of whether to shut down all those open applications or leave the lot running in the background. Obviously if there was no performance penalty, that latter would, of course, be preferable. It's a lot less hassle. Problem is, if you try that with just 2GB of RAM in Vista, you'll pay a pretty severe penalty.