I agree with the above; it sounds like you need more RAM. 450 MB at idle only gives you 550 MB to work with, which isn't really enough for huge maps. Another 1 GB stick ought to be $30 or so, so I'd go ahead and get it before upgrading to a new system - ought to solve all your problems for a very low cost.
PIV = Pentium IV.
I assume you have a desktop rather than a laptop, but I thought I'd check just in case. Even the best laptops are miserable for gaming (and anything else computationally intensive), mostly due to the hampered cooling functionality.
You obviously haven't looked at the laptop market recently. I can play every game out there except one at moderate or higher settings with my laptop, and it's a good deal off top of the line now. The best desktops will outperform the best laptops, true, but the gap isn't anywhere near as large as it used to be. Here's the comparisons of the best desktop and laptop hardware:
Graphics
Desktop: nvidia 8800 GTX.
3dmark06: 10763
Laptop: nvidia 8800M GTX
3dmark06: 9137
Processor
Desktop: Core 2 Quad 9650, 3.00 GHz, 12 MB L2 cache
Laptop: Core 2 Duo T9500, 2.60 GHz, 6 MB L2 cache
High-Wattage Laptop: Core 2 Extreme X9000, 2.8 GHz, 6 MB L2 cache
Also note that some laptops use desktop processors, making them equal in base power (if more limited in overclocking).
Memory for laptops is capped at 4 GB, but that's enough for nearly everything now. Hard drives aren't quite as fast, but laptops also have SSD options if you've got the money.
Even with far more moderate specs, as I have, you can still play 99% of all games out there at moderate or higher settings on a laptop. Integrated graphics laptops aren't that great, but if you buy the right parts, laptop gaming comes quite close to desktop gaming in performance.