When the wired and active RAM pass 50% - most tasks are measurably slower. You will see increased paging activity (vm_stat 15 in the terminal - watch for page outs - they indicate some memory had to be written to slow hard drive and constant page out and page in above 10-20 per minute indicates you have a RAM starved system.)
When starved for RAM, you can buy more or do less. You might want to get a monitoring software like Activity Monitor and keep the memory indicator in your dock. The system is likely thrashing based on the time delays you report. Each of the active programs needs some memory that just got paged to disk to free up extra space for memory needed by another program. It is a bad cycle and the computer just spends time running circles rather than getting done the work you wanted.
Here is a mac with every indication of RAM pressure:
But, quitting a large app like Aperture and starting it again still has no paging and the system is very responsive. Ignore the first line after the headings in vm_stat as it's the total since boot, but do watch for pageout as you have apps that are not responsive:
mac:~ me$ vm_stat 15
Mach Virtual Memory Statistics: (page size of 4096 bytes, cache hits 0%)
free active spec inactive wire faults copy 0fill reactive pageins pageout
260482 1186K 74303 275275 234520 323478K 2276158 188709K 120292 640158 482
260689 1187K 74303 275275 234038 4117 0 3256 0 0 0
261310 1185K 74001 275250 235835 10668 77 7115 0 4 0
292087 1153K 74711 274313 236770 32927 972 21684 0 18 0
521469 928347 75754 269861 235897 77208 130 63300 8 41 0
517897 936518 75766 269215 231981 27678 2504 13059 0 22 0
463894 987202 72076 274852 233371 126855 2712 89422 2 50 0
223701 1180K 77834 283568 265396 589512 255 562957 1 7278 0
205013 1186K 82765 291561 265293 389442 255 369664 0 7334 0
201401 1173K 89671 301672 265595 495426 82 472316 1 9286 0
However, starting about 4 of the Adobe CS4 apps can push this machine into thrashing and the slowness you describe with the apps taking 30 seconds to launch and lots of bouncing of square icons in the dock.
Here is the associated vm_stat 15 showing the problem. Look specifically at the free memory plummeting to the minimum and pagein / pageout climbing.
mac:~ me$ vm_stat 15
Mach Virtual Memory Statistics: (page size of 4096 bytes, cache hits 0%)
free active spec inactive wire faults copy 0fill reactive pageins pageout
21822 1190K 4746 585056 228411 332621K 2315577 196418K 121439 781551 1806
86862 1152K 5123 570803 215710 124555 932 102117 0 2086 15
73080 1161K 8631 571301 216607 24404 337 14931 0 1994 0
29655 1196K 13500 572024 219284 188966 535 112913 0 3606 0
1668 1208K 9616 594723 216954 174145 215 144996 16 15268 121
4142 1206K 5795 595393 219564 322750 0 320426 0 1 0
3043 1204K 3485 596990 223264 138981 136 136653 0 28 0
2338 1205K 2782 598556 222416 332263 321 321321 0 164 30
3251 1204K 2919 598770 222027 90135 82 85050 0 5 0
2606 1203K 212 600436 224678 114502 1 112450 0 35 0
1188 1200K 174 600230 228582 162660 117 157230 33127 860 9914
22069 1200K 27612 550547 230304 89953 38 82383 8274 159 2071
1910 1200K 349 600071 227976 24382 0 23465 7332 1 2985
1164 1201K 174 600281 228423 4940 1 4046 7087 8 2500
30404 1203K 186 567418 229475 16087 1 16045 8566 4 2235
It's like upgrade your house to have another room. Wether or not it justify's its cost depends on wether you use it or not. In general, I'd say 4GB is the minimum to go by these days, I'd opt for 8GB for most non-casual users.
Considering how expensive a MacBook is, I'd suggest going the extra mile to ensure your purchase lasts you well into the future. If your model is not a retina model (I.e. the RAM is user serviceable), I'd strongly recommend a 2x8GB kit like this one. At $130, it's only $30 more expensive than the 8GB Apple offers, but is DOUBLE the memory. (it's faster too, but your MacBook will bottleneck the speed difference, so it won't actually count for anything)
Best Answer
It's probably just been a while since you restarted your computer. When you close a program, the memory it was using doesn't go back to "Free", it gets marked "Inactive". It's still available for use by other programs, though: