Re: WinXP 32bit + XFast RAM = ...GB
In theory yes. So long as it matches what you have now. Caveats apply, read below.
The ram you next purchase must be an exact match SPD programming wise to your existing ram. If it isn't I will almost guarantee problems.
If you think matching will be hard, for whatever reason, IMHO you'd be best served by buying an 8Gb(2x4Gb) kit and shelving the current 2x2Gb you have.
Mine: ASRock Z77 Extreme6, i7-3770K, 16GB Samsung MV-3V4G3D/US, SeaSonic X-750 Gold, On-die HD 4000, Corsair H80i, Antec Spot Cool, Fractal Design R4 Black Pearl w/Window
Hers: ASRock 990FX Extreme4, FX-8150, 32GB AMD/Patriot AE316G1601U2K, SeaSonic X-750 Gold, Asus HD7770-2GD5, Corsair H80i, Antec Spot Cool, Fractal Design R4 Black Pearl w/Window
Server: Gigabyte GA-990FXA-UD5, FX-4170, 32GB AMD/Patriot AE316G1601U2K, Enermax ELT500AWT, IBM ServeRAID M1015, Sapphire Ultimate HD 4670, Norco RPC-470