Generally, graphics applications do benefit from larger memory spaces. You could put the extra 2 GB into the machine as it stands and XP 32 bit will recognise about up to 3.5 GB of it. That might be all you need and certainly would be the cheapest option - but no loss if you subsequently decide to go 64 bit.
a 64 bit system will give you a much bigger address space, with 4 GB real, a virtual address space of 8-10 GB should be very practical.
Before you take that step, though, do heed Bob's warning about drivers for ALL your devices. Run the Windows 7 Upgrade Advisor (free download from Microsoft) with all your devices attached and powered on. Then check out the manufacturers' web sites for updated drivers for any devices the advisor flags up. It will also check your applications - Adobe Photoshop and Acrobat need to be at pretty much the latest levels and they aren't exactly cheap!
If you do decide to go 64 bit, it's a no brainer - go Windows 7 - you really don't want to mess with XP 64 bit!
You also mentioned that saving your big files is very slow - 64 bit won't necessarily help the actual saving, though it may help in the generation of the file to be saved. You didn't mention what hard disk you have - if it's an older PATA or SATA 1, you might want to consider upgrading to a SATA 2 (add-in cards are available if your motherboard doesn't support it).
You might even want to consider a solid state hard "disk" - Kingston, Intel and others are now offering models that won't break the bank.