My cheepo computer that I would build today would be a board with 512mb of PC133 and an Athlon XP. With the Athlon 1600+ and 2100+ being the best buys. Not too many boards run pc133 and Athlons but they're out there. One of my computers has 512mb/2100 DDR. But 256mb/pc133 modules can be gotten cheap and even cheaper on sale.
Once we reached the 1GHz mark, it seems the actual speed increase became more 'on paper', as in, SiSoft, 3DMarks, PCMarks benchmark results.
Did you see much difference between 512 and one gig of memory? With windows 2000 or XP?
That would depend entirely on what you're doing with the computer: if you are running Enterprise sized databases, or rendering abnormally large graphics files, you would see an increase in performance however, it would be slight because at that point you're usually maxing out the CPU and you probably wouldn't be multi-tasking anyway.
For day to day use, such as web browsing and working on Office documents, you'd find no perceivable increase. Although, you'd notice less swapping when returning to the OS from games - if you were running huge-environment games like Battlefield 1942. But for running games like Tribes II or even Americas Army, you'd see very little improvement in swapping back to the OS.
Where you would really see a speed increase would be if you were to upgrade the Motherboard to a DDR400 or DDR333 memory system. Going from a PC133 system to a DDR400 system is amazing but, would probably be a waste of money for the average user...currently, I'd only suggest that kind of upgrade for a gamer or graphics artist.
Let's say you were working on 800 Mb of data, perhaps a big 3D model, or a very high res image, or a monster spread sheet, or indexing your Magnum Opus in Microsoft Word. With 512 Mb of memory, you're running at disk speeds, which is perhaps 10 milleseconds per block (seek time). With 1 Gb memory, you're running at memory speeds, which is perhaps 100 microseconds (roughly 1000 loads from 100 nanosecond memory).
That's the difference of a 100 to 1.
For accessing a single word, main memory is perhaps 100,000 times faster than disk.
So don't ask how much faster a system with 1 Gb is than a system with 512 Mb. Rather ask how much memory your application and data need, and size your system to have that much.
Having more memory than you need makes almost no difference at all. Having less memory than you need, even just a little bit less, can cause huge performance losses.