Will adding an extra 1gig of DDR ram help much in my case??

My current pc specs are: -

p4 2.0ghz @ 2.4ghz Northwood

Leadtek 7800gs 

2x512mb ddr pc2700 ram 

Windows XP pro SP2

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I am not planning on upgrading my PC till probably the end of the year now, the market is kinda unstable and i want to wait for the next gen AMD processors also to see what they offer in comparison to Intel.

On my aging gigabyte sis-645 motherboard i have room for 1 more ram stick.  Since im not upgrading my full pc till much later i was thinking of adding 1 gig for DDR ram, roughly £25 off of EBay so pricing is good. 

Seeing as my graphics card is clearly bottlenecked by my CPU, how much of a performance boost or load time reduction would adding an extra 1 gig of ram make to games like Quake 4 or C&C3.   

 Any help is greatly appreciated.

Thanks for the replies. 

£25 isnt really back breaking costs so a gain for that much would surely be a good thing.  Adding more ram does seem to be the next logical upgrade to my current setup.  As mentioned before im not intending to upgrade to a new machine till the end of the year.  Also if i was to change the CPU (besides its not the greatest time to upgrade when theres mega competition brewing!) i would need a new motherboard/ram etc etc...effectively will cost more than £25 lol.  Also saving £25 for something im intending to build 6 months down the line isnt really smart saving imo :P

I have vista business (free from my university:D) but not installing it on this setup because as deadmeat mentioned my cpu would cripple the overall performance.

Thanks for the advice posted, any further messages will also be greatly appreciated.