U already have 512mb in dual channel which is more than enough.
Addin 512mb wouldn't make much of a difference.
U have 512mb with a peak bandwidth of 6.4Gbps compared to 3.2GBps of single channel.
Peak Data bandwidth=(Memory speed)x(no of bytes transferred per channel)x(no of channels)
=400MHZ x 8 bytes x 2 channels
U should b really happy with that.
P4 1.6GHZ Willamette core 400MHZ FSB 256KB L2 cache
Intel 845WN Mobo
Nvidia Riva Tnt2 M64 G-card
40GB Seagate 5400rpm HDD
15 LG Studioworks563N
Creative Inspire2400 2.1 speakers
16x DVD ROM
52x Samsung CD Burner