What happens if I install ram that has a higher bus speed than the CPU?

Hello,

I'm hoping this question will be reasonably obvious to those in the know.

My old PC dates probably from around 2004.

It has an ASRock 4145GV Rev 5.01 motherboard and an Intel Celeron 2.6 Ghtz CPU. The OS is winXP Home SP3.

Now I'm up to Firefox 6, the old pensioner is not just stumbling, but falling over frequently. Once I open a few tabs, the wait can get very frustrating.

I maintain the system fairly well and have even disabled non-essential services, but the slowness is now ridiculous! Past experience suggests a memory upgrade could offer a new lease of life, especially as it currently has a pitiful 256Mb.

The motherboard has 2 slots for 184 pin ram. It claims to accept DDR1 and DDR2 up to a maximum of 2GB. Originally, I considered buying the full 2GB, but looking at the crucial website, it suggests that my useage only requires 1Gb. I reckon adding 1x 1Gb stick to the empty slot will be plenty with the existing 256Mb and if not, then I'll buy the other Gb.

The manual for the board states it will accept PC2700 (DDR333) and PC2100 (DDR266), with a caution: PC2700 is supported at FSB, 533Mhtz.

I didn't know what this was and the manual didn't say. A bit of googling reveals this is the Front Side Bus speed and is to do with the CPU. The CPU is a Northwood model with a bus speed of 400Mhtz.

I can get 1 Gb stick of PC2700 new for £15. PC2100 seems harder to find and is surprisingly more expensive - about twice as much it seems after a cursory glance and that's refurbished!

I have seen a second hand Pentium 4 (2.6Ghtz, FSB 800Htz) for £5.

What I want is to spend £15 and just clip the memory in and go. Some posts on the web suggest the memory will simply work at the speed of the FSB, some seem to suggest BIOS fiddling is required. I'm sure I could upgrade the CPU, but it's a pain and will the old fan fit or will I then find I need a new fan etc, etc.?

I expect there will be a slight performance benefit from the P4, although it's the same speed as the Celeron.

Please advise my best choice:
1. The PC2700 will work (but only at 400MHtz). £15
2. Pentium 4 is an easy upgrade to make the new memory work. (£20+ fiddling)
3. Pentium 4 will also need new fan and heatsink - Just pay for the older memory (£50+)

I'm hoping someone will assure me that option 1 is OK. If so, do I need to alter any settings such as multipliers in the BIOS after installation? Or are there any similar considerations with the other options?

Considering option 2, I looked for P4's that were faster (why not, if upgrading anyway?) and suddenly the price was £145 - perhaps these are still relatively current.

Thanks for any help.

-Tim