Plenty of drawbacks. By default, Windows will allocate a 4GB memory pool to every process, which is split 50/50 between the kernel mode processes (common to all apps) and user mode processes (unique for each app) (simplified explanation). An app running on the system therefore has 2GB of memory to play with, while the system itself has it's own 2GB. Important note: this second 2GB is the same 2GB for all apps running on the system.
The /3GB switch adjusts the split so that kernel mode gets 1GB and user mode gets 3GB.
Now consider the apps you're running. Some of them will require more kernel mode space, some will require more user mode space. As the kernel mode pool is shared, you can very quickly run out of memory there if you're running apps that put kernel mode memory under pressure. On the other hand, if your apps use lots of user mode memory, implementing /3GB will give them the headroom they require.
So it's really down to the nature of the applications you want to run. The golden rule is to consult the app vendor and read the documentation; in particular if the app vendor doesn't have a recommendation either way you should start becoming suspicious... have they properly tested their app or not? This is basic stuff that every vendor should know.
There's a pretty good discussion of it all here: http://blogs.technet.com/askperf/archive/2007/03/23/memory-management-demystifying-3gb.aspx
In your particular case, I think switching to 64 bit and getting more RAM will be a more viable solution, as /3GB won't really get you what you want (does it even work on XP?)