Hmm - a huge 'maybe' (maybe can help / maybe bad or good idea...). This is a maybe and it is not the best idea sometime (in recent years). RAM should be matched RAM and not mixed with others, assuming they are even of the same type. This creates performance problems with mixing/matching them. Personally, I remove all old and replace with all new and from the same lot for best performance gains and least memory errors.
Before you proceed, first identify the very reason your dell is slow, as you see it. Slow is relative but you need to determine you are short on RAM before thinking you need to add more. Example: its like adding more seats to your car to make it go faster -- not exactly the solution if speed is the problem (but can be related).
How to check RAM in your dell.
- start your computer into windows
- right click the task bar and choose Task Manager
- click the performance tab
- look at the graph in the lower left corner (memory).
*If it is full (lit up to the top), you are low on memory and adding more can help!
*if it is only half way up or certainly a good distance from the top, then adding more may not help.
General rule about amount of RAM in windows.
WinXP -- use 2 gig to 4 gig of Ram
WinVista -- 4gig - 6gig
Win7 -- 4gig - 16 gig (though you may never use more than 6 gig for any normal usage!)
Those OSes will run on far less ram but what I listed is for optimal performance. Also, keep in mind that your hard disk drive may need to run a CheckDisk (C > properties > tools > check now), your drive may need to be defragmented (c > properties > tools > defrag), your OS may need to be re-installed (windows registry gets silly, frequently, and a reinstall is the only solution for a registry DB problem), you may need to free up some drive space. If windows has less than 15%-20% free space on the C partition, it will run slower.
Also check your Internet connection. Liek example is if your slow computer is while using the internet and you have wireless than it could be a networking problem, not even a computer problem at all.
Basically - identify the "reason" it is slow before throwing band-aids at it. Then you should have better results.