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i3 is an Intel "low-end" processor range.
i5 is an Intel "mid-range" processor range.
i7 is an Intel "high-performance" processor range.
Generally speaking, the i7 is better (faster, more cache), then i5 and the i3 being "worst" (slower, less cache), but obviously the cost scales as well.
Some Wikipedia links you might find useful regarding these ranges: Core i3, Core i5, Core i7.
If you're interested in high-end games, remember that your GPU (Graphics card/adapter/etc) will be vitally important as well, as will having a reasonable amount of RAM. Having a massively powerful CPU will not help if your GPU is poor - you will still receive relatively poor gaming performance.
Using CS5 and some multimedia tools also leans me towards saying that you ensure you have a good quantity of RAM as well - these types of software can consume a lot of memory when running, and if you have insufficient RAM and they have to start using the page file then performance will be poor.
In summary, having an i5 over and i3 might help, it might not - it depends on the rest of the machine in question (you need to identify the weakest component - the performance bottleneck - and improve that for the best overall improvement).
@jtbandes & @DMA57361 thanks.. i am looking at laptops.. with config, i3/i5 processor, 4gb RAM, 1GB ATI Graphics card. will there be a very big difference in performance from i3 to i5? – The Learner – 2010-08-09T10:44:00.563
1@The Learner - The performance difference between the processors will depend most what speed they operate at and and the number of cores they have (the cache sizes are useful, but will have a less obvious effect). I believe all of the i3's are dual-core, so if you're looking at a quad-core i5 then it should perform multiple tasks better (note that more parallelism does not guarantee a single program will go faster, they might - depends on how they were written and what they do). – DMA57361 – 2010-08-09T11:16:19.897
3Whilst it is a good answer, I want to add than even the Core I3 is a very good CPU - it is only poor in direct comparison to the I5 or I7, if you are upgrading from anything over 3-4 years old, (Celeron or P4 generation) - you will see vastly improved performance. – William Hilsum – 2010-08-22T14:43:00.827
1@Wil a very valid point. All references to performance in my answer are relative comparisons between the i[3|5|7] processors. This isn't particular obviously from my post, so good catch! – DMA57361 – 2010-08-22T14:55:52.567
1No problem and still up voted your answer! just wanted it to be clear incase anyone Googles/comes across this answer and thinks that the I3 is bad! – William Hilsum – 2010-08-22T15:06:20.267
There are different types of i3 and i5 processors. What computer are you looking at specifically? – jtbandes – 2010-08-09T07:21:36.240