with large databases you'll want as much memory as you can afford

6 to 8 gigs would be a good place to start.
(you'll need a 64 bit OS to support that much ram NOT XP - windows 7 is out on pre-order)

also its all about access time, RAID 0 would be a good thing to have(but have good backups)

graphics card isn't going to do anything for you but pick up a cheap PCI-E cause an onboard is not very good and it will eat some of your system ram.

Thanks for that :) I was wondering if a 64 bit OS would improve things - I'll hold off until Windows 7 arrives.

If you are planning to use more than 3GB of memory, 64bit Windows is mandatory.

I'm not sure where the threshold is for Linux, but it is theoretically impossible to access more than 4GB of memory without a 64bit OS.

Kaled.

The memory limits page on the Microsoft website [msdn.microsoft.com] will show you that you can use more than 4GB physically memory effectively on many 32 bit versions of their operating systems. Even the old Windows 2000 Server OS is capable of addressing 32 GB in the Datacenter version.

The main problem is not addressing that amount of memory with a 32 bit OS. The 8 bit 8088 and 16 bit 8086 processors were already capable of addressing 1MB of memory with a 16 bit OS. The problem is that with the flat memory model most 32 bit programs are programmed in, that any process is only capable of addressing 4GB. With some memory reserved for accessing hardware like videocards, this leaves an effective 3GB per process.

But with the right 32bit Windows OS, you are capable of using more than 4GB physical memory, as long as it is divided over multiple processes:

3GB for the SQL database process
3GB as disk cache
etc.

While 32 bit can be divided into effective use, those 3gb limits remain. Since this is a new machine do go with a new 64 bit OS and a large as you can do (affordably) the first time around. Saves heartache later when further hardware upgrades become necessary. And always remember that even though new hardware can help, bad coding practices and failure to streamline all functions can negate any advance in speed/capacity! (Been there, done that, learned the hard way!)

O/S
I don't know about Windows 7 but you can near enough double the speed of a new Vista PC with basic optimising.

Named brand PCs will be heavily loaded with utilities, free software etc. All those fight for resources - especially at startup.

Vista has to be the most insecure O/S ever. I don't mean as in security but in the way it looks after itself. Vista has OCD.

Vista tries to confirm, re-affirm and log EVERYTHING it does It even logs when it is logging. No joke.

If you turn off most of the logs, unecessary services, tune the memory you could see a doubling in speed.

Disks
Consider SSD. These are much cheaper and more reliable now and will really make a difference.

Note that there are some older generation SSD out there so make sure you have the latest (read / write speeds of around 200MB/s, the older types are around 90 to 150 MB/s)

Get a normal hard disk just for backups.

Memory
As already stated you will be limited here by the O/S. If 32 bit and XP or Vista you won't get more than around 3.5Gb to work.

Go for a motherboard with the latest DDR3 memory. This will be the fastest / give best bandwidth.

Graphics
You can get PCs with onboard graphics but this is not ideal as unless it has it's own dedicated memory it will take RAM. Best option is to purchase separately a cheap PCI Express card for around $40.

I take it the Home Premium version of Windows 7 would be fine?