Minecraft.exe is the launcher, not Minecraft itself; to allocate more RAM, click on "edit profile" in the launcher, then near the bottom check "JVM arguments" and put what you want in there; it will automatically be applied when you start the game. See this screenshot.
Also, unless you are using big modpacks or HD texture packs or run 32 bit Java/OS there is no need to mess around with the JVM arguments; Mojang put them there for a reason, especially to help 1.8 run better (all the "-XX:+UseConcMarkSweepGC" and stuff; for 32 bit is is probably a good idea to reduce the memory allocated; however, even 512 MB is enough for vanilla, even 1.8, on normal settings).
I remember doing the same thing to play on a modpack years ago, but now it won't work for some reason. please please halp.
P.S.: Don't mind the last sentence in portuguese, I'm brazilian. it means "Press any key to continue"
Also, unless you are using big modpacks or HD texture packs or run 32 bit Java/OS there is no need to mess around with the JVM arguments; Mojang put them there for a reason, especially to help 1.8 run better (all the "-XX:+UseConcMarkSweepGC" and stuff; for 32 bit is is probably a good idea to reduce the memory allocated; however, even 512 MB is enough for vanilla, even 1.8, on normal settings).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?