Thanks to your advice, I have already purchased the Kingston 2GB kit, I will try it out. So far with 1GB of RAM I do not have any OOM errors. Some CTD sometimes (fe.dll or g3d.dll modules) while being in O'Hare. Let's hope this boost of RAM will help eliminate the CTD's I have at O'Hare and won't create new ones at the upcoming JFK.
If you have the budget you might want to upgrade the PC overall. Get new CPU, Mobo and DDR2 RAM. It's a shame if you spend more money on this PC while you can collect more money and do the big upgrade. But as I said it depends on your budget and needs.
Budget for a new machine, not a problem, I could buy a SLI mobo with a Core 2 Quad, 2x Geforce GTX 280 & 2x WD Velociraptor 300GB in RAID-0 tomorrow if I wanted to but the thing is, I have spent so much time building my FS2004 configuration the way it is right now (many add-on payware aircrafts, sceneries, etc.), it runs well on this system and I really enjoy it. I will probably skip Vista and wait for the new OS to come out... (I am testing Win Vista right now at the workplace and I don't really like it) Plus a new system would mean: reformat, installing FSX (Vista & DX10 combination). Staying on this system saves me all this hassle and I perform great backups for this config in case a disaster would occur.