Ummm.. you alread have a gig of
RAM-- another gig would be great though.

and that would max your system-- so you'd never have to buy it again... for this PC anyway
You could add a second Harddrive-- maybe move your primary install to the larger new one-- use the old one for backups/music storage
The Video card...I would keep the one you have-- the reason being that Motherboard only supports up to AGP 4X-- (The higher standard is 8X) so spending a lot on a video card won't gain much-- it's the interface built into the board. Even 8X is considered obsolete.
PCI express is the new format. Requires a motherboard/chip/
ram/videocard upgrade
The Monitor-- I would suggest checking your video card specs to see if it can handle a wide-screen monitor-- they use odd resolutions-- which may not be supported by the card
Save your money and change out the Motherboard /Processor/
Ram/Video card later on-- upgrade to PCI express. This will require backup of all your personal stuff and reinstallation of Operating Systems (why I suggested the
second harddrive)
You will also be able to buy them as a "combo" package-- saving some cash-- and a bit of time for installation .Out of box-into computer-- hook up the "main" drive + Power Supply Unit-- start installing Windows. Then hook up your backup drive-- and you still have all your stuff.