Yeah Rdram PC800 is still very good indeed, certainly as good as DDR PC2700. 256MB is not bad at all but adding between 128MB and 256MB is worth it esp if you run WinXP. Skt423 is still good and won't choke even a 2ghz CPU, but you do pay more for the slower Skt423 CPU. If an adapter will make your mobo Skt478 (minus the 533FSB ability I assume) it may be worth considering BUT at $50 it's as much as a very good and very new TRUE Skt478 mobo. Add to the fact that DDR PC2700 is cheap (compared to any Rambus) and it should be very tempting for you to consider switching to a Skt478 NW P4, 533FSB (either by default or o/c'ing) and 512MB DDR PC2700. I'd suggest you add up the cost and the pro/cons for each option open to you. Compared even to some PIII and Duron systems a P4 1.5ghz is certainly 'showing its age' so do consider your options carefully. Another thing to consider is switching to AMD which would really keep the cost down, AthlonXP2000+ is VERY cheap and nicely powerful (certainly equal to P4 2ghz) and would work VERY well even with DDR2100 although PC2700 would be better considering the price diff is small. Anyway there are plenty of options open to you so be sure to explore them all, if you want a quick, simple boost then a gfx card alone WILL give you PC a nice boost, IMHO you should be considering a CPU and mobo upgrade in the next few months but you can always take a new gfx card over to the 'new' PC anyway. Considering you're using a GF2MX even a GF4MX440 or Rad9000PRO will give a BIG boost, although GF4TI4200 or Rad9500PRO are worth the extra cash (esp if you plan to add a faster CPU later). If you plan to keep your 1.5ghz then you could save by going for a Rad8500LE-128MB (or other 8500) as this is a nice match and a very good gfx card!
I know a review site did a perf of a 4600 and 9700PRO on a range of CPUs (both P4 and A-XP) and this included a P4 1.6ghz, I'll route around, Xbit or FiringSquad IIRC.
HERE it is