I own the exact same model and I can tell you that Gateway's specs are wrong- the D945GCL board does support 4GB ram (2x2GB). I know, because I researched the heck out of it AND that's what I'm using. I bought ADATA ram from Newegg- it's DDR2-800. Yes, the board is limited to DDR2-667 speeds, but the sticks were the same price, the board just drops them automatically to 667, and you may upgrade someday to a (used) PC that can utilize the 800 speed.
Even if you stick with Vista 32, it's still worth it to buy 4GB of ram; it may only see 3-3.5GB, but matching sticks run in dual channel, which helps. Plus, I found that buying one 2GB and one 1GB stick cost the same as a matched 2x2GB pair- so why not? If anything, the DDR2-800 will be more desirable to someone on craigslist when it's time to dump them... don't get rid of the original ram that came with your GT5404!
This advice may come too late for you, but I'm providing it for others to hopefully save them all the countless hours I spent researching this. If you're still in doubt (which you shouldn't be), go to Crucial.com (a memory selling site) and use their computer scanner to verify this. It will scan your mobo (with your permission, of course) and tell you how much ram it can handle. The beauty of this test is that you don't tell the scanner what equipment you have- it tells you. This way, it's not just giving recommendations based on the erroneous specs provided by Gateway. I checked MANY memory selling sites- where you tell them what make and model you have, and 50% of them were wrong and said 2GB max.
Whatever ram you buy for the D945GCL, it MUST match these specs:
1) NON-ECC ONLY (Speaking from experience here- ECC will NOT work)
2) DDR2-ANY Pricing is surprisingly similar for all speeds, so get the highest. Think resale value!
3) 1.8 volts only- no more, no less. Fortunately this is a very common voltage for DDR2.
Finally- if you haven't purchased Windows 7 yet, I probably wouldn't. My GT5404 "sees" 3.3GBs outta 4, and gaining the extra .7GB isn't worth the cost. Also, there will be TONS of Vista bashers who will insist that Windows 7 will crush Vista in terms of speed... but remember, this is a five year old pc, and it can only do so much. Although the D915 cpu looks good on paper, in reality it's not so great. Sure, 2.8GHZ and dual core sounds pretty competitive even today, but this cpu is based on 65nm tech, which is pretty ancient now. Put it this way- a core2duo rated at 1.8MHZ runs circles around it... clock speeds only tell part of the story.
Good Luck!