Vista and RAM

How mush RAM is a good amount when running Vista? I'm running 2 Corsair ddr2 XMS2 Pc5400* 512's (or 1Gb). Vista seems to be eating my ram, so wht sould i get?

(CPU= C2Duo E6300, VideoCard= BFG tech 7600 GT)

(*might be Pc5200, cant remember)

get 2x more 512MBs of that corsair ram, here CORSAIR XMS2 1GB (2 x 512MB) 240-Pin DDR2 SDRAM DDR2 675 (PC2 5400) Dual Channel Kit Desktop Memory Model TWIN2X1024-5400C4 - Retail

I only have 2 DDR2 slots in my motherboard, (it's a cheap motherboard, but it works), so i was thinking about getting either Corsair 2Gb (2 x 1Gb) ddr2 800 Pc2 6400, or Corsair 2Gb (2 x 1Gb) ddr2 675 Pc2 5400. The Pc2 6400 is 116.00 (only 86.00 right now after mail in rebate), but the Pc2 5400 is only 69.99. Is it worth it for the Pc2 6400? Will there be a difference really?

http://www.newegg.com/Product/Product.aspx?Item=N82E16820145590 - Pc2 6400

http://www.newegg.com/Product/Product.aspx?Item=N82E16820145015 -Pc2 5400